AUAudioOutputStream::Stop() first sets the source_ to NULL then calls...

AUAudioOutputStream::Stop() first sets the source_ to NULL then calls AudioOutputUnitStop(), since AudioOutputUnitStop() is called by AudioManager thread, it will do a synchronous stop which means that AUAudioOutputStream::Render() will keep running until it is done. so that it crashes when it hits source_ which has been set to NULL. The solution is to stop the HAL first and then set the source_ to NULL.

BUG=120193
TEST=None
TAB=tommi

Review URL: https://chromiumcodereview.appspot.com/9834098

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@129590 0039d316-1c4b-4281-b951-d872f2087c98