mac: Fix hover highlight regression for background tabs.
mac: Fix hover highlight regression for background tabs.

Background tabs should show an alpha-blended version of the foreground tab
background on hover. https://codereview.chromium.org/11638011 regressed this
by drawing the background bitmap as highlight.

BUG=52468
TEST=Install a theme where foreground and background tabs look very different.
Hover over a background tab, observe visible highlight.


Review URL: https://chromiumcodereview.appspot.com/11710005

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@174777 0039d316-1c4b-4281-b951-d872f2087c98

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