Allow shaped-desktop hosts to send shape only when it changes.

remoting/codec/video_decoder_verbatim.cc

Index: remoting/codec/video_decoder_verbatim.cc
diff --git a/remoting/codec/video_decoder_verbatim.cc b/remoting/codec/video_decoder_verbatim.cc
index 05af0ceeaba2825a2ad47b285dd872f134ed0324..dd8c2f331d8cd84ce515a27136fcec053f9db4a9 100644
--- a/remoting/codec/video_decoder_verbatim.cc
+++ b/remoting/codec/video_decoder_verbatim.cc
@@ -108,7 +108,7 @@ void VideoDecoderVerbatim::RenderFrame(const webrtc::DesktopSize& view_size,
 }
 
 const webrtc::DesktopRegion* VideoDecoderVerbatim::GetImageShape() {
-  return NULL;
+  return nullptr;
 }
 
 }  // namespace remoting