Add Chrome tracing to buck launcher
Add Chrome tracing to buck launcher

Summary:
This extends Chrome tracing out into the Python buck launcher.
To use the tracer, just write:

  from tracing import Tracing

  with Tracing('eventname'):
    pass

Then, at the end of your run:

  Tracing.write_to_dir(buck_log_dir)

We currently write to buck-out/log/traces/launch.[uuid].trace and make
a symlink from launch.trace to that file.

Test Plan:
Added tracing to buck's main() method, ran 'buck project',
confirmed trace was written to buck-out/log/traces/launch.[uuid].trace
and symlink existed. Opened in chrome://tracing in Chrome, confirmed
timestamps made sense and matched build.trace.

zhuxiaohao/buck,Heart2009/buck,Learn-Android-app/buck,davido/buck,marcinkwiatkowski/buck,romanoid/buck,zpao/buck,ilya-klyuchnikov/buck,raviagarwal7/buck,illicitonion/buck,hgl888/buck,ilya-klyuchnikov/buck,mogers/buck,vschs007/buck,sdwilsh/buck,darkforestzero/buck,k21/buck,vine/buck,rowillia/buck,shs96c/buck,pwz3n0/buck,Learn-Android-app/buck,raviagarwal7/buck,pwz3n0/buck,daedric/buck,zhan-xiong/buck,tgummerer/buck,dushmis/buck,grumpyjames/buck,shybovycha/buck,hgl888/buck,mread/buck,MarkRunWu/buck,robbertvanginkel/buck,artiya4u/buck,rmaz/buck,shybovycha/buck,1yvT0s/buck,davido/buck,rowillia/buck,shybovycha/buck,pwz3n0/buck,vschs007/buck,daedric/buck,mnuessler/buck,bocon13/buck,artiya4u/buck,Distrotech/buck,MarkRunWu/buck,vine/buck,shybovycha/buck,janicduplessis/buck,LegNeato/buck,OkBuilds/buck,Dominator008/buck,sdwilsh/buck,darkforestzero/buck,tgummerer/buck,robbertvanginkel/buck,facebook/buck,shs96c/buck,bocon13/buck,shybovycha/buck,LegNeato/buck,tgummerer/buck,robbertvanginkel/buck,clonetwin26/buck,davido/buck,siddhartharay007/buck,luiseduardohdbackup/buck,luiseduardohdbackup/buck,justinmuller/buck,shs96c/buck,mread/buck,marcinkwiatkowski/buck,illicitonion/buck,ilya-klyuchnikov/buck,davido/buck,rmaz/buck,justinmuller/buck,janicduplessis/buck,zhan-xiong/buck,raviagarwal7/buck,facebook/buck,Heart2009/buck,nguyentruongtho/buck,luiseduardohdbackup/buck,hgl888/buck,marcinkwiatkowski/buck,darkforestzero/buck,Learn-Android-app/buck,facebook/buck,rmaz/buck,stuhood/buck,Addepar/buck,Distrotech/buck,romanoid/buck,darkforestzero/buck,marcinkwiatkowski/buck,janicduplessis/buck,daedric/buck,mogers/buck,SeleniumHQ/buck,clonetwin26/buck,daedric/buck,marcinkwiatkowski/buck,lukw00/buck,davido/buck,darkforestzero/buck,Heart2009/buck,clonetwin26/buck,illicitonion/buck,vine/buck,liuyang-li/buck,artiya4u/buck,zhan-xiong/buck,kageiit/buck,LegNeato/buck,stuhood/buck,SeleniumHQ/buck,OkBuilds/buck,mikekap/buck,zhuxiaohao/buck,dpursehouse/buck,artiya4u/buck,robbertvanginkel/buck,mogers/buck,stuhood/buck,Dominator008/buck,dpursehouse/buck,kageiit/buck,bocon13/buck,grumpyjames/buck,mikekap/buck,OkBuilds/buck,vschs007/buck,sdwilsh/buck,justinmuller/buck,dsyang/buck,brettwooldridge/buck,sdwilsh/buck,raviagarwal7/buck,dushmis/buck,JoelMarcey/buck,rowillia/buck,OkBuilds/buck,dushmis/buck,Addepar/buck,hgl888/buck,clonetwin26/buck,ilya-klyuchnikov/buck,stuhood/buck,luiseduardohdbackup/buck,tgummerer/buck,sdwilsh/buck,janicduplessis/buck,luiseduardohdbackup/buck,marcinkwiatkowski/buck,facebook/buck,zpao/buck,zpao/buck,MarkRunWu/buck,mnuessler/buck,Heart2009/buck,clonetwin26/buck,1yvT0s/buck,k21/buck,Dominator008/buck,mread/buck,dsyang/buck,rmaz/buck,stuhood/buck,OkBuilds/buck,mikekap/buck,zpao/buck,vine/buck,darkforestzero/buck,mread/buck,k21/buck,romanoid/buck,mnuessler/buck,liuyang-li/buck,clonetwin26/buck,shs96c/buck,ilya-klyuchnikov/buck,mnuessler/buck,grumpyjames/buck,zhan-xiong/buck,sdwilsh/buck,Dominator008/buck,artiya4u/buck,SeleniumHQ/buck,illicitonion/buck,zhan-xiong/buck,neonichu/buck,marcinkwiatkowski/buck,rhencke/buck,mikekap/buck,rhencke/buck,k21/buck,JoelMarcey/buck,darkforestzero/buck,marcinkwiatkowski/buck,daedric/buck,Heart2009/buck,mogers/buck,brettwooldridge/buck,bocon13/buck,shybovycha/buck,Learn-Android-app/buck,liuyang-li/buck,bocon13/buck,pwz3n0/buck,hgl888/buck,darkforestzero/buck,mogers/buck,luiseduardohdbackup/buck,vine/buck,OkBuilds/buck,liuyang-li/buck,zhuxiaohao/buck,brettwooldridge/buck,siddhartharay007/buck,zhan-xiong/buck,kageiit/buck,rowillia/buck,clonetwin26/buck,LegNeato/buck,JoelMarcey/buck,grumpyjames/buck,artiya4u/buck,neonichu/buck,Dominator008/buck,illicitonion/buck,shs96c/buck,rhencke/buck,SeleniumHQ/buck,stuhood/buck,k21/buck,stuhood/buck,romanoid/buck,dpursehouse/buck,JoelMarcey/buck,shs96c/buck,SeleniumHQ/buck,mnuessler/buck,brettwooldridge/buck,bocon13/buck,lukw00/buck,vschs007/buck,OkBuilds/buck,brettwooldridge/buck,shybovycha/buck,LegNeato/buck,liuyang-li/buck,vine/buck,shybovycha/buck,dushmis/buck,shybovycha/buck,brettwooldridge/buck,ilya-klyuchnikov/buck,zhuxiaohao/buck,shybovycha/buck,vschs007/buck,kageiit/buck,bocon13/buck,dpursehouse/buck,grumpyjames/buck,rhencke/buck,grumpyjames/buck,robbertvanginkel/buck,Dominator008/buck,janicduplessis/buck,neonichu/buck,Dominator008/buck,nguyentruongtho/buck,Distrotech/buck,darkforestzero/buck,romanoid/buck,rmaz/buck,shs96c/buck,Heart2009/buck,zhuxiaohao/buck,Dominator008/buck,JoelMarcey/buck,justinmuller/buck,siddhartharay007/buck,clonetwin26/buck,stuhood/buck,JoelMarcey/buck,1yvT0s/buck,neonichu/buck,nguyentruongtho/buck,artiya4u/buck,romanoid/buck,dsyang/buck,dsyang/buck,1yvT0s/buck,SeleniumHQ/buck,pwz3n0/buck,mogers/buck,illicitonion/buck,rowillia/buck,kageiit/buck,MarkRunWu/buck,rhencke/buck,zhan-xiong/buck,JoelMarcey/buck,Addepar/buck,daedric/buck,zhan-xiong/buck,vschs007/buck,zhuxiaohao/buck,vschs007/buck,vine/buck,shs96c/buck,OkBuilds/buck,Dominator008/buck,shs96c/buck,liuyang-li/buck,rmaz/buck,vschs007/buck,kageiit/buck,clonetwin26/buck,siddhartharay007/buck,tgummerer/buck,vine/buck,davido/buck,mikekap/buck,marcinkwiatkowski/buck,k21/buck,Distrotech/buck,brettwooldridge/buck,brettwooldridge/buck,neonichu/buck,mikekap/buck,brettwooldridge/buck,SeleniumHQ/buck,janicduplessis/buck,Addepar/buck,raviagarwal7/buck,Heart2009/buck,mnuessler/buck,siddhartharay007/buck,tgummerer/buck,rowillia/buck,marcinkwiatkowski/buck,bocon13/buck,robbertvanginkel/buck,artiya4u/buck,lukw00/buck,SeleniumHQ/buck,janicduplessis/buck,illicitonion/buck,neonichu/buck,darkforestzero/buck,daedric/buck,dsyang/buck,Distrotech/buck,dpursehouse/buck,Dominator008/buck,Learn-Android-app/buck,JoelMarcey/buck,janicduplessis/buck,rowillia/buck,rhencke/buck,justinmuller/buck,illicitonion/buck,clonetwin26/buck,clonetwin26/buck,zhan-xiong/buck,robbertvanginkel/buck,rhencke/buck,dsyang/buck,SeleniumHQ/buck,Addepar/buck,rmaz/buck,Distrotech/buck,stuhood/buck,shybovycha/buck,Learn-Android-app/buck,rmaz/buck,tgummerer/buck,ilya-klyuchnikov/buck,robbertvanginkel/buck,vschs007/buck,LegNeato/buck,Addepar/buck,raviagarwal7/buck,vschs007/buck,raviagarwal7/buck,janicduplessis/buck,bocon13/buck,bocon13/buck,Addepar/buck,rowillia/buck,robbertvanginkel/buck,justinmuller/buck,justinmuller/buck,robbertvanginkel/buck,Dominator008/buck,LegNeato/buck,SeleniumHQ/buck,dushmis/buck,mread/buck,raviagarwal7/buck,Addepar/buck,ilya-klyuchnikov/buck,mogers/buck,luiseduardohdbackup/buck,dsyang/buck,rmaz/buck,rhencke/buck,justinmuller/buck,MarkRunWu/buck,k21/buck,Addepar/buck,liuyang-li/buck,pwz3n0/buck,neonichu/buck,liuyang-li/buck,vschs007/buck,Learn-Android-app/buck,mikekap/buck,daedric/buck,rmaz/buck,SeleniumHQ/buck,luiseduardohdbackup/buck,siddhartharay007/buck,bocon13/buck,brettwooldridge/buck,rowillia/buck,ilya-klyuchnikov/buck,MarkRunWu/buck,zpao/buck,shs96c/buck,marcinkwiatkowski/buck,mikekap/buck,tgummerer/buck,marcinkwiatkowski/buck,robbertvanginkel/buck,Addepar/buck,Addepar/buck,Learn-Android-app/buck,mikekap/buck,dpursehouse/buck,tgummerer/buck,davido/buck,dpursehouse/buck,grumpyjames/buck,rowillia/buck,ilya-klyuchnikov/buck,dpursehouse/buck,grumpyjames/buck,brettwooldridge/buck,1yvT0s/buck,pwz3n0/buck,artiya4u/buck,dsyang/buck,tgummerer/buck,justinmuller/buck,vschs007/buck,brettwooldridge/buck,clonetwin26/buck,liuyang-li/buck,davido/buck,Learn-Android-app/buck,zhuxiaohao/buck,OkBuilds/buck,lukw00/buck,k21/buck,mogers/buck,Distrotech/buck,justinmuller/buck,dsyang/buck,hgl888/buck,mnuessler/buck,davido/buck,rmaz/buck,zhan-xiong/buck,ilya-klyuchnikov/buck,grumpyjames/buck,vine/buck,rhencke/buck,janicduplessis/buck,davido/buck,neonichu/buck,romanoid/buck,JoelMarcey/buck,nguyentruongtho/buck,zhuxiaohao/buck,rhencke/buck,dushmis/buck,MarkRunWu/buck,justinmuller/buck,Addepar/buck,bocon13/buck,grumpyjames/buck,k21/buck,daedric/buck,grumpyjames/buck,darkforestzero/buck,raviagarwal7/buck,hgl888/buck,facebook/buck,mikekap/buck,k21/buck,Distrotech/buck,pwz3n0/buck,siddhartharay007/buck,daedric/buck,vschs007/buck,facebook/buck,sdwilsh/buck,rmaz/buck,mikekap/buck,stuhood/buck,dushmis/buck,pwz3n0/buck,dsyang/buck,k21/buck,sdwilsh/buck,nguyentruongtho/buck,SeleniumHQ/buck,darkforestzero/buck,romanoid/buck,daedric/buck,1yvT0s/buck,luiseduardohdbackup/buck,rowillia/buck,LegNeato/buck,OkBuilds/buck,MarkRunWu/buck,shybovycha/buck,k21/buck,liuyang-li/buck,ilya-klyuchnikov/buck,tgummerer/buck,k21/buck,mogers/buck,lukw00/buck,SeleniumHQ/buck,grumpyjames/buck,lukw00/buck,shybovycha/buck,lukw00/buck,dsyang/buck,romanoid/buck,siddhartharay007/buck,romanoid/buck,LegNeato/buck,vine/buck,JoelMarcey/buck,LegNeato/buck,zhan-xiong/buck,lukw00/buck,illicitonion/buck,zhuxiaohao/buck,rowillia/buck,mnuessler/buck,shs96c/buck,raviagarwal7/buck,shs96c/buck,Learn-Android-app/buck,darkforestzero/buck,OkBuilds/buck,1yvT0s/buck,stuhood/buck,nguyentruongtho/buck,LegNeato/buck,mnuessler/buck,lukw00/buck,zpao/buck,vine/buck,liuyang-li/buck,daedric/buck,davido/buck,lukw00/buck,kageiit/buck,justinmuller/buck,illicitonion/buck,lukw00/buck,hgl888/buck,dushmis/buck,sdwilsh/buck,JoelMarcey/buck,robbertvanginkel/buck,janicduplessis/buck,robbertvanginkel/buck,justinmuller/buck,artiya4u/buck,davido/buck,zpao/buck,siddhartharay007/buck,sdwilsh/buck,davido/buck,zhan-xiong/buck,romanoid/buck,sdwilsh/buck,pwz3n0/buck,romanoid/buck,mogers/buck,Learn-Android-app/buck,sdwilsh/buck,Heart2009/buck,raviagarwal7/buck,ilya-klyuchnikov/buck,Dominator008/buck,hgl888/buck,clonetwin26/buck,mikekap/buck,pwz3n0/buck,facebook/buck,shs96c/buck,illicitonion/buck,1yvT0s/buck,janicduplessis/buck,romanoid/buck,mogers/buck,OkBuilds/buck,neonichu/buck,raviagarwal7/buck,artiya4u/buck,Distrotech/buck,sdwilsh/buck,zhan-xiong/buck,rhencke/buck,JoelMarcey/buck,LegNeato/buck,1yvT0s/buck,marcinkwiatkowski/buck,JoelMarcey/buck,mread/buck,dsyang/buck,dushmis/buck,brettwooldridge/buck,raviagarwal7/buck,LegNeato/buck,daedric/buck,OkBuilds/buck,Distrotech/buck,illicitonion/buck,rmaz/buck,Addepar/buck,Distrotech/buck,nguyentruongtho/buck,dushmis/buck,Heart2009/buck,illicitonion/buck,dsyang/buck