Adding opus tests to media browsertests

Adding tests for opus in media_browsertest and
media_source_browsertests.

test=content_browsertests

Review URL: https://codereview.chromium.org/114833002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241386 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241386