Don't return NULL from AudioManager:GetMessageLoop()
Don't return NULL from AudioManager:GetMessageLoop()

Returning NULL leads to a bunch of hairy corner cases where
other clients on the audio thread need to constantly check if
the message loop is still valid.

Returning NULL from GetMessageLoop seems to be a relic from
the past, so for don't do it anymore.  If we see browser
shutdown hangs in the field we can revert and consider
alternate solutions.

BUG=157957
TEST=all tests still pass.

Review URL: https://chromiumcodereview.appspot.com/11348016

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@165638 0039d316-1c4b-4281-b951-d872f2087c98

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