Remove theoretical race condition between Stop() and Initialize().
Remove theoretical race condition between Stop() and Initialize().

Initialize() could acquire lock_ and launch a new thread_ while Stop() had
un-lock_'d and was joining the old thread.  If that happened then Stop() would
silently leak the new thread when it re-acquired lock_ and set thread_ to NULL.

This also removes a cheap but unnecessary lock/unlock pair and clarifies the
function a bit.

Tested: built & ran media_unittests.

Patch by fischman@chromium.org:
http://codereview.chromium.org/6531012/

TEST=none
BUG=none

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@75212 0039d316-1c4b-4281-b951-d872f2087c98

dushu1203/chromium.src,timopulkkinen/BubbleFish,Fireblend/chromium-crosswalk,robclark/chromium,M4sse/chromium.src,anirudhSK/chromium,ChromiumWebApps/chromium,chuan9/chromium-crosswalk,junmin-zhu/chromium-rivertrail,keishi/chromium,mogoweb/chromium-crosswalk,rogerwang/chromium,M4sse/chromium.src,junmin-zhu/chromium-rivertrail,jaruba/chromium.src,mohamed--abdel-maksoud/chromium.src,ondra-novak/chromium.src,littlstar/chromium.src,Fireblend/chromium-crosswalk,nacl-webkit/chrome_deps,bright-sparks/chromium-spacewalk,Fireblend/chromium-crosswalk,rogerwang/chromium,pozdnyakov/chromium-crosswalk,ondra-novak/chromium.src,dednal/chromium.src,patrickm/chromium.src,littlstar/chromium.src,hgl888/chromium-crosswalk,mogoweb/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,krieger-od/nwjs_chromium.src,bright-sparks/chromium-spacewalk,krieger-od/nwjs_chromium.src,dednal/chromium.src,rogerwang/chromium,ChromiumWebApps/chromium,M4sse/chromium.src,anirudhSK/chromium,zcbenz/cefode-chromium,chuan9/chromium-crosswalk,Pluto-tv/chromium-crosswalk,zcbenz/cefode-chromium,dushu1203/chromium.src,ChromiumWebApps/chromium,ChromiumWebApps/chromium,patrickm/chromium.src,mohamed--abdel-maksoud/chromium.src,ChromiumWebApps/chromium,krieger-od/nwjs_chromium.src,Chilledheart/chromium,axinging/chromium-crosswalk,rogerwang/chromium,M4sse/chromium.src,krieger-od/nwjs_chromium.src,TheTypoMaster/chromium-crosswalk,crosswalk-project/chromium-crosswalk-efl,zcbenz/cefode-chromium,jaruba/chromium.src,mogoweb/chromium-crosswalk,littlstar/chromium.src,Jonekee/chromium.src,ondra-novak/chromium.src,dushu1203/chromium.src,TheTypoMaster/chromium-crosswalk,zcbenz/cefode-chromium,dednal/chromium.src,Jonekee/chromium.src,bright-sparks/chromium-spacewalk,jaruba/chromium.src,robclark/chromium,dushu1203/chromium.src,markYoungH/chromium.src,M4sse/chromium.src,nacl-webkit/chrome_deps,patrickm/chromium.src,keishi/chromium,TheTypoMaster/chromium-crosswalk,fujunwei/chromium-crosswalk,mohamed--abdel-maksoud/chromium.src,crosswalk-project/chromium-crosswalk-efl,M4sse/chromium.src,robclark/chromium,axinging/chromium-crosswalk,hgl888/chromium-crosswalk-efl,axinging/chromium-crosswalk,mogoweb/chromium-crosswalk,rogerwang/chromium,markYoungH/chromium.src,crosswalk-project/chromium-crosswalk-efl,dushu1203/chromium.src,dednal/chromium.src,timopulkkinen/BubbleFish,hgl888/chromium-crosswalk-efl,ondra-novak/chromium.src,crosswalk-project/chromium-crosswalk-efl,krieger-od/nwjs_chromium.src,ondra-novak/chromium.src,hgl888/chromium-crosswalk,mohamed--abdel-maksoud/chromium.src,hgl888/chromium-crosswalk,dushu1203/chromium.src,mohamed--abdel-maksoud/chromium.src,PeterWangIntel/chromium-crosswalk,krieger-od/nwjs_chromium.src,pozdnyakov/chromium-crosswalk,timopulkkinen/BubbleFish,nacl-webkit/chrome_deps,bright-sparks/chromium-spacewalk,Fireblend/chromium-crosswalk,Jonekee/chromium.src,keishi/chromium,patrickm/chromium.src,keishi/chromium,anirudhSK/chromium,Fireblend/chromium-crosswalk,dednal/chromium.src,junmin-zhu/chromium-rivertrail,junmin-zhu/chromium-rivertrail,ondra-novak/chromium.src,bright-sparks/chromium-spacewalk,patrickm/chromium.src,fujunwei/chromium-crosswalk,Just-D/chromium-1,rogerwang/chromium,TheTypoMaster/chromium-crosswalk,ltilve/chromium,zcbenz/cefode-chromium,Pluto-tv/chromium-crosswalk,markYoungH/chromium.src,markYoungH/chromium.src,Pluto-tv/chromium-crosswalk,hgl888/chromium-crosswalk-efl,anirudhSK/chromium,timopulkkinen/BubbleFish,patrickm/chromium.src,ondra-novak/chromium.src,markYoungH/chromium.src,hujiajie/pa-chromium,rogerwang/chromium,hgl888/chromium-crosswalk-efl,zcbenz/cefode-chromium,Pluto-tv/chromium-crosswalk,hujiajie/pa-chromium,Pluto-tv/chromium-crosswalk,timopulkkinen/BubbleFish,rogerwang/chromium,pozdnyakov/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,hujiajie/pa-chromium,pozdnyakov/chromium-crosswalk,Jonekee/chromium.src,nacl-webkit/chrome_deps,mogoweb/chromium-crosswalk,littlstar/chromium.src,Chilledheart/chromium,fujunwei/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,ltilve/chromium,timopulkkinen/BubbleFish,junmin-zhu/chromium-rivertrail,Jonekee/chromium.src,robclark/chromium,M4sse/chromium.src,junmin-zhu/chromium-rivertrail,pozdnyakov/chromium-crosswalk,dushu1203/chromium.src,Just-D/chromium-1,zcbenz/cefode-chromium,fujunwei/chromium-crosswalk,dednal/chromium.src,pozdnyakov/chromium-crosswalk,timopulkkinen/BubbleFish,hgl888/chromium-crosswalk,fujunwei/chromium-crosswalk,axinging/chromium-crosswalk,M4sse/chromium.src,Just-D/chromium-1,Just-D/chromium-1,ltilve/chromium,hgl888/chromium-crosswalk-efl,jaruba/chromium.src,Jonekee/chromium.src,PeterWangIntel/chromium-crosswalk,krieger-od/nwjs_chromium.src,keishi/chromium,fujunwei/chromium-crosswalk,Chilledheart/chromium,Pluto-tv/chromium-crosswalk,jaruba/chromium.src,littlstar/chromium.src,hgl888/chromium-crosswalk-efl,Just-D/chromium-1,pozdnyakov/chromium-crosswalk,chuan9/chromium-crosswalk,anirudhSK/chromium,ltilve/chromium,junmin-zhu/chromium-rivertrail,dushu1203/chromium.src,junmin-zhu/chromium-rivertrail,ChromiumWebApps/chromium,ChromiumWebApps/chromium,zcbenz/cefode-chromium,anirudhSK/chromium,mohamed--abdel-maksoud/chromium.src,TheTypoMaster/chromium-crosswalk,rogerwang/chromium,jaruba/chromium.src,Jonekee/chromium.src,hgl888/chromium-crosswalk,littlstar/chromium.src,mogoweb/chromium-crosswalk,keishi/chromium,fujunwei/chromium-crosswalk,Chilledheart/chromium,ltilve/chromium,nacl-webkit/chrome_deps,junmin-zhu/chromium-rivertrail,axinging/chromium-crosswalk,hujiajie/pa-chromium,Chilledheart/chromium,Just-D/chromium-1,Pluto-tv/chromium-crosswalk,littlstar/chromium.src,axinging/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,zcbenz/cefode-chromium,TheTypoMaster/chromium-crosswalk,patrickm/chromium.src,Jonekee/chromium.src,hgl888/chromium-crosswalk-efl,hgl888/chromium-crosswalk,dednal/chromium.src,timopulkkinen/BubbleFish,hgl888/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,jaruba/chromium.src,Chilledheart/chromium,junmin-zhu/chromium-rivertrail,Just-D/chromium-1,ChromiumWebApps/chromium,ondra-novak/chromium.src,hgl888/chromium-crosswalk-efl,PeterWangIntel/chromium-crosswalk,markYoungH/chromium.src,hgl888/chromium-crosswalk,Jonekee/chromium.src,robclark/chromium,ChromiumWebApps/chromium,chuan9/chromium-crosswalk,anirudhSK/chromium,mogoweb/chromium-crosswalk,anirudhSK/chromium,nacl-webkit/chrome_deps,ltilve/chromium,fujunwei/chromium-crosswalk,M4sse/chromium.src,dushu1203/chromium.src,dushu1203/chromium.src,timopulkkinen/BubbleFish,chuan9/chromium-crosswalk,hujiajie/pa-chromium,pozdnyakov/chromium-crosswalk,robclark/chromium,mohamed--abdel-maksoud/chromium.src,markYoungH/chromium.src,M4sse/chromium.src,ltilve/chromium,keishi/chromium,chuan9/chromium-crosswalk,anirudhSK/chromium,hujiajie/pa-chromium,patrickm/chromium.src,keishi/chromium,Pluto-tv/chromium-crosswalk,nacl-webkit/chrome_deps,Fireblend/chromium-crosswalk,zcbenz/cefode-chromium,Fireblend/chromium-crosswalk,pozdnyakov/chromium-crosswalk,hujiajie/pa-chromium,bright-sparks/chromium-spacewalk,mogoweb/chromium-crosswalk,hgl888/chromium-crosswalk-efl,ltilve/chromium,krieger-od/nwjs_chromium.src,krieger-od/nwjs_chromium.src,zcbenz/cefode-chromium,patrickm/chromium.src,pozdnyakov/chromium-crosswalk,krieger-od/nwjs_chromium.src,hujiajie/pa-chromium,keishi/chromium,ondra-novak/chromium.src,axinging/chromium-crosswalk,hgl888/chromium-crosswalk,hujiajie/pa-chromium,littlstar/chromium.src,Chilledheart/chromium,chuan9/chromium-crosswalk,axinging/chromium-crosswalk,Jonekee/chromium.src,Just-D/chromium-1,crosswalk-project/chromium-crosswalk-efl,Chilledheart/chromium,chuan9/chromium-crosswalk,anirudhSK/chromium,mogoweb/chromium-crosswalk,junmin-zhu/chromium-rivertrail,robclark/chromium,jaruba/chromium.src,mohamed--abdel-maksoud/chromium.src,crosswalk-project/chromium-crosswalk-efl,jaruba/chromium.src,nacl-webkit/chrome_deps,ltilve/chromium,ChromiumWebApps/chromium,fujunwei/chromium-crosswalk,Jonekee/chromium.src,Fireblend/chromium-crosswalk,robclark/chromium,rogerwang/chromium,markYoungH/chromium.src,krieger-od/nwjs_chromium.src,Chilledheart/chromium,mohamed--abdel-maksoud/chromium.src,axinging/chromium-crosswalk,robclark/chromium,Pluto-tv/chromium-crosswalk,markYoungH/chromium.src,anirudhSK/chromium,jaruba/chromium.src,hujiajie/pa-chromium,crosswalk-project/chromium-crosswalk-efl,dushu1203/chromium.src,keishi/chromium,hgl888/chromium-crosswalk-efl,M4sse/chromium.src,timopulkkinen/BubbleFish,dednal/chromium.src,hujiajie/pa-chromium,markYoungH/chromium.src,keishi/chromium,PeterWangIntel/chromium-crosswalk,chuan9/chromium-crosswalk,timopulkkinen/BubbleFish,robclark/chromium,nacl-webkit/chrome_deps,anirudhSK/chromium,mohamed--abdel-maksoud/chromium.src,ChromiumWebApps/chromium,bright-sparks/chromium-spacewalk,crosswalk-project/chromium-crosswalk-efl,ChromiumWebApps/chromium,axinging/chromium-crosswalk,nacl-webkit/chrome_deps,dednal/chromium.src,PeterWangIntel/chromium-crosswalk,axinging/chromium-crosswalk,Fireblend/chromium-crosswalk,nacl-webkit/chrome_deps,dednal/chromium.src,mohamed--abdel-maksoud/chromium.src,PeterWangIntel/chromium-crosswalk,mogoweb/chromium-crosswalk,dednal/chromium.src,Just-D/chromium-1,markYoungH/chromium.src,bright-sparks/chromium-spacewalk,crosswalk-project/chromium-crosswalk-efl,bright-sparks/chromium-spacewalk,pozdnyakov/chromium-crosswalk,jaruba/chromium.src