Clean up the audio timestamp and duration code.
Clean up the audio timestamp and duration code.

While debugging this code I found it to be a tangled web of logic.  The new version is much simpler:
  - Always calculate a duration (this makes sense anyway)
  - Use the estimated timestamp when one isn't available
  - Reset the estimated timestamp when a valid timestamp is available

BUG=26036
TEST=everything still plays in sync

Review URL: http://codereview.chromium.org/1695009

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@45339 0039d316-1c4b-4281-b951-d872f2087c98

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