media: Adding Opus support in WebM for <video> tag

      In earlier CLs, support for playing back Opus in WebM container
was added to Media Source. This CL tries to implement the same
spec so that Opus in WebM can be played back in <video> tag too.

Note: This depends on an ffmpeg roll being done first (it's being worked on). This CL should not be committed until that's resolved.

TEST=media_unittests

Review URL: https://codereview.chromium.org/23868037

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232484 0039d316-1c4b-4281-b951-d872f2087c98