cc: Video layers without a frame should not occlude

cc/layers/video_frame_provider.h

Index: cc/layers/video_frame_provider.h
diff --git a/cc/layers/video_frame_provider.h b/cc/layers/video_frame_provider.h
index ff632f6546dea421dac7d7e9d6d2ddf07aa7711a..aa6a821636c2fffbf7be7fe297e14969dfca8fb3 100644
--- a/cc/layers/video_frame_provider.h
+++ b/cc/layers/video_frame_provider.h
@@ -67,6 +67,10 @@ class CC_EXPORT VideoFrameProvider {
   virtual bool UpdateCurrentFrame(base::TimeTicks deadline_min,
                                   base::TimeTicks deadline_max) = 0;
 
+  // Returns true if GetCurrentFrame() will return a non-null frame and false
+  // otherwise. Aside from thread locks, the state won't change.
+  virtual bool HasCurrentFrame() = 0;
+
   // Returns the current frame, which may have been updated by a recent call to
   // UpdateCurrentFrame(). A call to this method does not ensure that the frame
   // will be rendered. A subsequent call to PutCurrentFrame() must be made if