Add presubmit check for correct MessageLoopProxy usage in src/media/.

In most cases SingleThreadTaskRunner can be used instead.

BUG=315922
R=dalecurtis@chromium.org

Review URL: https://codereview.chromium.org/101503007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243408 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243408