Change VideoDecoder::ReadCB to take const scoped_refptr<VideoFrame>&.

BUG=none
TEST=media_unittests, media layout tests.


Review URL: https://chromiumcodereview.appspot.com/10559074

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@143192 0039d316-1c4b-4281-b951-d872f2087c98
  virtual ~RTCVideoDecoderTest() {
    message_loop_.RunAllPending();
  }

  virtual ~RTCVideoDecoderTest() {
    message_loop_.RunAllPending();
  }
