Add thread safety to AudioManagerBase to protect access to the audio thread member variable.
This is is a tentative fix for an issue where the AudioManager can crash while tearing down
the audio thread. I suspect that this happens because more than one cleanup attempts were
made since the class wasn't thread safe. I'm also changing direct access to the thread's
MessageLoop* to use MessageLoopProxy based on the same theory.
BUG=110051
TEST=Run media tests.
Review URL: https://chromiumcodereview.appspot.com/9255017
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@118272 0039d316-1c4b-4281-b951-d872f2087c98
16 files changed
