Bug 1346120 part 4 - Only set ImageContainer if there are valid new images in VideoSink::RenderVideoFrames(); r=jwwang

So that the suspended video element won't be rendered any more and keeps the last decoded frame.
This is the effect that UX specification defines.
And actually, we don't need to set ImageContainer if there are no valid new images.

MozReview-Commit-ID: B7RS3LXu8J0

--HG--
extra : rebase_source : de7d014ebad34f032a0ea8bfbe9aa723ffe922de
extra : source : d6a2b47b14f6ac00ea420f5eba7190c7af725381

dom/media/mediasink/VideoSink.cpp

@@ -400,7 +400,10 @@ VideoSink::RenderVideoFrames(int32_t aMaxFrames,
     VSINK_LOG_V("playing video frame %" PRId64 " (id=%x) (vq-queued=%" PRIuSIZE ")",
                 frame->mTime, frame->mFrameID, VideoQueue().GetSize());
   }
-  mContainer->SetCurrentFrames(frames[0]->As<VideoData>()->mDisplay, images);
+
+  if (images.Length() > 0) {
+    mContainer->SetCurrentFrames(frames[0]->As<VideoData>()->mDisplay, images);
+  }
 }
 
 void