Update VideoFramePump to pass un-changed frames to encoders.

remoting/codec/video_encoder_verbatim.cc

Index: remoting/codec/video_encoder_verbatim.cc
diff --git a/remoting/codec/video_encoder_verbatim.cc b/remoting/codec/video_encoder_verbatim.cc
index c7550e1713397f9d836ac71d439d4641b4e31046..2948e3e27128bba6f78c31cfcd72b436a70df488 100644
--- a/remoting/codec/video_encoder_verbatim.cc
+++ b/remoting/codec/video_encoder_verbatim.cc
@@ -25,7 +25,12 @@ VideoEncoderVerbatim::~VideoEncoderVerbatim() {}
 
 scoped_ptr<VideoPacket> VideoEncoderVerbatim::Encode(
     const webrtc::DesktopFrame& frame) {
-  CHECK(frame.data());
+  DCHECK(frame.data());
+
+  // If nothing has changed in the frame then return NULL to indicate that
+  // we don't need to actually send anything (e.g. nothing to top-off).
+  if (frame.updated_region().is_empty())
+    return nullptr;
 
   base::Time encode_start_time = base::Time::Now();