Use base::MessageLoopProxy instead of MessageLoop* in webkit/media/.

Minor cleanup split off into a separate CL while working on bug 177730.

Review URL: https://codereview.chromium.org/12388039

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@185400 0039d316-1c4b-4281-b951-d872f2087c98
5 files changed