Fix DesktopSessioAgent crash when handling nullptr OnCaptureCompleted callback.

remoting/host/desktop_session_agent.cc

Index: remoting/host/desktop_session_agent.cc
diff --git a/remoting/host/desktop_session_agent.cc b/remoting/host/desktop_session_agent.cc
index e7ab06325e9342f3678ff5b944fc7bf0399f4dc7..abf9cbccfd6ef379a99e88a5c5843d545a323d2d 100644
--- a/remoting/host/desktop_session_agent.cc
+++ b/remoting/host/desktop_session_agent.cc
@@ -326,6 +326,10 @@ void DesktopSessionAgent::OnCaptureCompleted(webrtc::DesktopFrame* frame) {
 
   last_frame_.reset(frame);
 
+  if (!frame) {

[Sergey Ulanov, 2016/05/19 22:23:52]

I don't think we can just ignore the error here. The network process will still
think that capture request is pending. I think you want to send
ChromotingDesktopNetworkMsg_CaptureCompleted message with an empty
serialized_frame here. Then update DesktopSessionProxy::OnCaptureCompleted() to
handle empty serialized_frame.

[Hzj_jie, 2016/05/20 00:11:15]

Yes, I paused this review because of the screen frozen. Both DesktopSessionAgent
and DesktopSessionProxy need to be changed. And I am currently trying, if it
works well, I will restart the review.

+    return;
+  }
+
   current_size_ = frame->size();
 
   // Serialize webrtc::DesktopFrame.