Fix DesktopSessioAgent crash when handling nullptr OnCaptureCompleted callback.

remoting/host/desktop_session_agent.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "remoting/host/desktop_session_agent.h"
 
 #include <utility>
 
 #include "base/files/file_util.h"
 #include "base/logging.h"
@@ skipping 308 matching lines @@
           base::Bind(&DesktopSessionAgent::SendToNetwork, this))));
   mouse_cursor_monitor_ = desktop_environment_->CreateMouseCursorMonitor();
   mouse_cursor_monitor_->Init(this, webrtc::MouseCursorMonitor::SHAPE_ONLY);
 }
 
 void DesktopSessionAgent::OnCaptureCompleted(webrtc::DesktopFrame* frame) {
   DCHECK(caller_task_runner_->BelongsToCurrentThread());
 
   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.
   SerializedDesktopFrame serialized_frame;
   serialized_frame.shared_buffer_id = frame->shared_memory()->id();
   serialized_frame.bytes_per_row = frame->stride();
   serialized_frame.dimensions = frame->size();
   serialized_frame.capture_time_ms = frame->capture_time_ms();
   serialized_frame.dpi = frame->dpi();
   for (webrtc::DesktopRegion::Iterator i(frame->updated_region());
@@ skipping 239 matching lines @@
   }
 }
 
 void DesktopSessionAgent::StopAudioCapturer() {
   DCHECK(audio_capture_task_runner_->BelongsToCurrentThread());
 
   audio_capturer_.reset();
 }
 
 }  // namespace remoting