Report media source metrics independent of media measurement.

BUG=None
TEST=tools/perf/run_benchmark media.mse_cases

Review URL: https://codereview.chromium.org/117803004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242567 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242567