[Extension] Fix build failure on Windows
[Extension] Fix build failure on Windows

Use '\' for the product_dir on Windows in external_extension_sample.gyp.
Otherwise, the build system can't handle the path well in some cases,
e.g. creating or deleting them.
It fails the windows build when it starts from zero.

qjia7/crosswalk,bestwpw/crosswalk,minggangw/crosswalk,chuan9/crosswalk,seanlong/crosswalk,mrunalk/crosswalk,seanlong/crosswalk,zeropool/crosswalk,tedshroyer/crosswalk,marcuspridham/crosswalk,stonegithubs/crosswalk,weiyirong/crosswalk-1,qjia7/crosswalk,lincsoon/crosswalk,jondwillis/crosswalk,minggangw/crosswalk,shaochangbin/crosswalk,mrunalk/crosswalk,mrunalk/crosswalk,hgl888/crosswalk,DonnaWuDongxia/crosswalk,tedshroyer/crosswalk,jondong/crosswalk,qjia7/crosswalk,huningxin/crosswalk,alex-zhang/crosswalk,shaochangbin/crosswalk,lincsoon/crosswalk,Bysmyyr/crosswalk,pk-sam/crosswalk,jondong/crosswalk,lincsoon/crosswalk,DonnaWuDongxia/crosswalk,fujunwei/crosswalk,Bysmyyr/crosswalk,leonhsl/crosswalk,rakuco/crosswalk,darktears/crosswalk,rakuco/crosswalk,darktears/crosswalk,crosswalk-project/crosswalk,hgl888/crosswalk-efl,mrunalk/crosswalk,baleboy/crosswalk,dreamsxin/crosswalk,XiaosongWei/crosswalk,baleboy/crosswalk,Pluto-tv/crosswalk,chuan9/crosswalk,fujunwei/crosswalk,TheDirtyCalvinist/spacewalk,xzhan96/crosswalk,dreamsxin/crosswalk,tomatell/crosswalk,rakuco/crosswalk,zliang7/crosswalk,huningxin/crosswalk,darktears/crosswalk,DonnaWuDongxia/crosswalk,myroot/crosswalk,TheDirtyCalvinist/spacewalk,siovene/crosswalk,stonegithubs/crosswalk,hgl888/crosswalk,jondwillis/crosswalk,zliang7/crosswalk,Bysmyyr/crosswalk,crosswalk-project/crosswalk-efl,heke123/crosswalk,darktears/crosswalk,jondwillis/crosswalk,jondong/crosswalk,baleboy/crosswalk,lincsoon/crosswalk,leonhsl/crosswalk,Bysmyyr/crosswalk,hgl888/crosswalk,PeterWangIntel/crosswalk,axinging/crosswalk,ZhengXinCN/crosswalk,zeropool/crosswalk,PeterWangIntel/crosswalk,fujunwei/crosswalk,huningxin/crosswalk,dreamsxin/crosswalk,darktears/crosswalk,tedshroyer/crosswalk,heke123/crosswalk,DonnaWuDongxia/crosswalk,zliang7/crosswalk,crosswalk-project/crosswalk-efl,wuhengzhi/crosswalk,weiyirong/crosswalk-1,huningxin/crosswalk,baleboy/crosswalk,axinging/crosswalk,bestwpw/crosswalk,PeterWangIntel/crosswalk,marcuspridham/crosswalk,heke123/crosswalk,fujunwei/crosswalk,myroot/crosswalk,jpike88/crosswalk,hgl888/crosswalk-efl,baleboy/crosswalk,stonegithubs/crosswalk,heke123/crosswalk,marcuspridham/crosswalk,shaochangbin/crosswalk,amaniak/crosswalk,heke123/crosswalk,weiyirong/crosswalk-1,RafuCater/crosswalk,crosswalk-project/crosswalk,alex-zhang/crosswalk,DonnaWuDongxia/crosswalk,chuan9/crosswalk,amaniak/crosswalk,bestwpw/crosswalk,RafuCater/crosswalk,xzhan96/crosswalk,jpike88/crosswalk,zeropool/crosswalk,hgl888/crosswalk,mrunalk/crosswalk,Bysmyyr/crosswalk,XiaosongWei/crosswalk,jpike88/crosswalk,tomatell/crosswalk,wuhengzhi/crosswalk,bestwpw/crosswalk,Bysmyyr/crosswalk,jondong/crosswalk,Pluto-tv/crosswalk,fujunwei/crosswalk,PeterWangIntel/crosswalk,jondong/crosswalk,zliang7/crosswalk,pk-sam/crosswalk,hgl888/crosswalk-efl,myroot/crosswalk,seanlong/crosswalk,hgl888/crosswalk,hgl888/crosswalk,darktears/crosswalk,hgl888/crosswalk-efl,baleboy/crosswalk,leonhsl/crosswalk,chuan9/crosswalk,shaochangbin/crosswalk,rakuco/crosswalk,crosswalk-project/crosswalk,crosswalk-project/crosswalk,minggangw/crosswalk,tomatell/crosswalk,Pluto-tv/crosswalk,minggangw/crosswalk,darktears/crosswalk,heke123/crosswalk,bestwpw/crosswalk,jondong/crosswalk,crosswalk-project/crosswalk-efl,mrunalk/crosswalk,tomatell/crosswalk,siovene/crosswalk,siovene/crosswalk,jondong/crosswalk,myroot/crosswalk,XiaosongWei/crosswalk,axinging/crosswalk,jpike88/crosswalk,RafuCater/crosswalk,siovene/crosswalk,qjia7/crosswalk,baleboy/crosswalk,xzhan96/crosswalk,XiaosongWei/crosswalk,rakuco/crosswalk,jondwillis/crosswalk,XiaosongWei/crosswalk,chinakids/crosswalk,heke123/crosswalk,axinging/crosswalk,XiaosongWei/crosswalk,wuhengzhi/crosswalk,pk-sam/crosswalk,zliang7/crosswalk,chinakids/crosswalk,minggangw/crosswalk,crosswalk-project/crosswalk,leonhsl/crosswalk,weiyirong/crosswalk-1,marcuspridham/crosswalk,leonhsl/crosswalk,seanlong/crosswalk,huningxin/crosswalk,seanlong/crosswalk,tomatell/crosswalk,tedshroyer/crosswalk,axinging/crosswalk,hgl888/crosswalk,crosswalk-project/crosswalk-efl,wuhengzhi/crosswalk,rakuco/crosswalk,amaniak/crosswalk,xzhan96/crosswalk,zliang7/crosswalk,weiyirong/crosswalk-1,xzhan96/crosswalk,chuan9/crosswalk,stonegithubs/crosswalk,chinakids/crosswalk,jondwillis/crosswalk,dreamsxin/crosswalk,alex-zhang/crosswalk,TheDirtyCalvinist/spacewalk,crosswalk-project/crosswalk,pk-sam/crosswalk,fujunwei/crosswalk,qjia7/crosswalk,Pluto-tv/crosswalk,minggangw/crosswalk,tedshroyer/crosswalk,lincsoon/crosswalk,stonegithubs/crosswalk,ZhengXinCN/crosswalk,TheDirtyCalvinist/spacewalk,lincsoon/crosswalk,alex-zhang/crosswalk,minggangw/crosswalk,xzhan96/crosswalk,PeterWangIntel/crosswalk,minggangw/crosswalk,lincsoon/crosswalk,rakuco/crosswalk,jondwillis/crosswalk,bestwpw/crosswalk,jpike88/crosswalk,marcuspridham/crosswalk,RafuCater/crosswalk,DonnaWuDongxia/crosswalk,siovene/crosswalk,Bysmyyr/crosswalk,Bysmyyr/crosswalk,crosswalk-project/crosswalk,XiaosongWei/crosswalk,DonnaWuDongxia/crosswalk,jpike88/crosswalk,xzhan96/crosswalk,amaniak/crosswalk,marcuspridham/crosswalk,stonegithubs/crosswalk,jondong/crosswalk,RafuCater/crosswalk,PeterWangIntel/crosswalk,jpike88/crosswalk,chinakids/crosswalk,wuhengzhi/crosswalk,shaochangbin/crosswalk,chinakids/crosswalk,Pluto-tv/crosswalk,hgl888/crosswalk,baleboy/crosswalk,dreamsxin/crosswalk,ZhengXinCN/crosswalk,leonhsl/crosswalk,myroot/crosswalk,chuan9/crosswalk,tomatell/crosswalk,hgl888/crosswalk-efl,zeropool/crosswalk,chinakids/crosswalk,shaochangbin/crosswalk,hgl888/crosswalk-efl,marcuspridham/crosswalk,alex-zhang/crosswalk,marcuspridham/crosswalk,zeropool/crosswalk,ZhengXinCN/crosswalk,pk-sam/crosswalk,zeropool/crosswalk,TheDirtyCalvinist/spacewalk,amaniak/crosswalk,lincsoon/crosswalk,dreamsxin/crosswalk,jondwillis/crosswalk,xzhan96/crosswalk,ZhengXinCN/crosswalk,crosswalk-project/crosswalk-efl,crosswalk-project/crosswalk-efl,fujunwei/crosswalk,alex-zhang/crosswalk,ZhengXinCN/crosswalk,TheDirtyCalvinist/spacewalk,rakuco/crosswalk,siovene/crosswalk,dreamsxin/crosswalk,axinging/crosswalk,pk-sam/crosswalk,ZhengXinCN/crosswalk,stonegithubs/crosswalk,leonhsl/crosswalk,RafuCater/crosswalk,siovene/crosswalk,qjia7/crosswalk,Pluto-tv/crosswalk,heke123/crosswalk,zliang7/crosswalk,weiyirong/crosswalk-1,seanlong/crosswalk,PeterWangIntel/crosswalk,huningxin/crosswalk,weiyirong/crosswalk-1,Pluto-tv/crosswalk,amaniak/crosswalk,zliang7/crosswalk,tomatell/crosswalk,tedshroyer/crosswalk,pk-sam/crosswalk,axinging/crosswalk,crosswalk-project/crosswalk-efl,zeropool/crosswalk,amaniak/crosswalk,chuan9/crosswalk,wuhengzhi/crosswalk,tedshroyer/crosswalk,myroot/crosswalk,alex-zhang/crosswalk,darktears/crosswalk,bestwpw/crosswalk,RafuCater/crosswalk,hgl888/crosswalk-efl,crosswalk-project/crosswalk