Removes comment.

The comment refers to a field that does not exist.

Bug: 746182
Change-Id: I76586b938980f95e93a01386ea7bde7844d59f4d
Reviewed-on: https://chromium-review.googlesource.com/1087324Reviewed-by: Frank Liberato <liberato@chromium.org>
Commit-Queue: CJ DiMeglio <lethalantidote@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565032}

media/blink/video_frame_compositor.h

@@ -68,8 +68,6 @@ class MEDIA_BLINK_EXPORT VideoFrameCompositor : public VideoRendererSink,
 
   // |task_runner| is the task runner on which this class will live,
   // though it may be constructed on any thread.
-  // |media_context_provider_callback| requires being called on the media
-  // thread.
   VideoFrameCompositor(
       const scoped_refptr<base::SingleThreadTaskRunner>& task_runner,
       std::unique_ptr<blink::WebVideoFrameSubmitter> submitter);