message: Don't do ToI420() for each frame while checking is it dummy
  Cr-Commit-Position: refs/heads/master@{#28429}
diff: @@ -32,11 +32,11 @@ rtc::scoped_refptr<webrtc::VideoFrameBuffer> CreateDummyFrameBuffer() {
  bool IsDummyFrameBuffer(
  rtc::scoped_refptr<webrtc::VideoFrameBuffer> video_frame_buffer) {
  - rtc::scoped_refptr<webrtc::I420BufferInterface> buffer =
  - video_frame_buffer->ToI420();
  - if (buffer->width() != 2 || buffer->height() != 2) {
  + if (video_frame_buffer->width() != 2 || video_frame_buffer->height() != 2) {
  return false;
  }
  + rtc::scoped_refptr<webrtc::I420BufferInterface> buffer =
  + video_frame_buffer->ToI420();
  if (memcmp(buffer->DataY(), kIrrelatedSimulcastStreamFrameData, 2) != 0) {
  return false;
  }
  