Android: Delete placeholder texture when exiting remote playback.

Since OnDisconnectedFromRemoteDevice() releases WMP's reference
to the MailboxHolder, we'd get a TextureNoLongerNeeded callback
when the compositor is also done with it. We then just have to
be careful to do things on the right thread and not leak the texture.

NOTRY=True

BUG=284478

Review URL: https://codereview.chromium.org/27451004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230198 0039d316-1c4b-4281-b951-d872f2087c98
Remove --enable-audible-notifications flag for feature launch.

BUG=290550

Review URL: https://codereview.chromium.org/32543006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230199 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230199