Don't try to post tasks to a NULL MessageLoop from MediaLog.
Don't try to post tasks to a NULL MessageLoop from MediaLog.

(this situation can arise b/c VideoRendererBase uses a
base::PlatformThread::Delegate instead of a more-legit base::Thread)

BUG=93821
TEST=trybots

Review URL: http://codereview.chromium.org/7709015

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@97866 0039d316-1c4b-4281-b951-d872f2087c98

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