Allow individual volume control of remote webrtc audio tracks.
This fixes an issue where remote audio tracks would all share the same volume state.  Although there's still a limitation in the audio stack, this brings us a step closer to spec compliance.

BUG=343998
TEST=I ran the currently disabled, yet functional WebRtc tests in content_unittests.  Also created my own test HTML page that renders the same media stream in multiple video tags and verified that individual volume control worked and that the volume could be boosted up to a max factor of 10.
NOTRY=True

Review URL: https://codereview.chromium.org/135393009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251663 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251663