Update VideoFramePump to pass un-changed frames to encoders.

remoting/codec/video_encoder.h

Index: remoting/codec/video_encoder.h
diff --git a/remoting/codec/video_encoder.h b/remoting/codec/video_encoder.h
index f497eb3b4c3839b6acad5a7968d35f70c2ee8d04..9da447c1336f17394dd54b6d5273f81c4656f48a 100644
--- a/remoting/codec/video_encoder.h
+++ b/remoting/codec/video_encoder.h
@@ -25,7 +25,10 @@ class VideoEncoder {
   virtual void SetLosslessEncode(bool want_lossless) {}
   virtual void SetLosslessColor(bool want_lossless) {}
 
-  // Encode an image stored in |frame|.
+  // Encode an image stored in |frame|. If |frame.updated_region()| is empty
+  // then the encoder may return a packet (e.g. to top-off previously-encoded
+  // portions of the frame to higher quality) or return nullptr to indicate that
+  // there is no work to do.
   virtual scoped_ptr<VideoPacket> Encode(const webrtc::DesktopFrame& frame) = 0;
 };