Fix DesktopSessioAgent crash when handling nullptr OnCaptureCompleted callback.

remoting/host/desktop_session_proxy.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_proxy.h"
 
 #include <stddef.h>
 
 #include <utility>
 
@@ skipping 453 matching lines @@
   DCHECK(caller_task_runner_->BelongsToCurrentThread());
 
   // Drop the cached reference to the buffer.
   shared_buffers_.erase(id);
 }
 
 void DesktopSessionProxy::OnCaptureCompleted(
     const SerializedDesktopFrame& serialized_frame) {
   DCHECK(caller_task_runner_->BelongsToCurrentThread());
 
+  --pending_capture_frame_requests_;
+
+  // If the input serialized_frame does not have a screen size, it means the
+  // capturer returns a nullptr for OnCaptureCompleted call.
+  if (serialized_frame.dimensions.is_empty()) {
+    video_capturer_->OnCaptureCompleted(nullptr);
+    return;
+  }
+
   // Assume that |serialized_frame| is well-formed because it was received from
   // a more privileged process.
   scoped_refptr<IpcSharedBufferCore> shared_buffer_core =
       GetSharedBufferCore(serialized_frame.shared_buffer_id);
   CHECK(shared_buffer_core.get());
 
   std::unique_ptr<webrtc::DesktopFrame> frame(
       new webrtc::SharedMemoryDesktopFrame(
           serialized_frame.dimensions, serialized_frame.bytes_per_row,
           new IpcSharedBuffer(shared_buffer_core)));
   frame->set_capture_time_ms(serialized_frame.capture_time_ms);
   frame->set_dpi(serialized_frame.dpi);
 
   for (size_t i = 0; i < serialized_frame.dirty_region.size(); ++i) {
     frame->mutable_updated_region()->AddRect(serialized_frame.dirty_region[i]);
   }
 
-  --pending_capture_frame_requests_;
   video_capturer_->OnCaptureCompleted(std::move(frame));
 }
 
 void DesktopSessionProxy::OnMouseCursor(
     const webrtc::MouseCursor& mouse_cursor) {
   DCHECK(caller_task_runner_->BelongsToCurrentThread());
 
   if (mouse_cursor_monitor_) {
     mouse_cursor_monitor_->OnMouseCursor(
         base::WrapUnique(webrtc::MouseCursor::CopyOf(mouse_cursor)));
@@ skipping 26 matching lines @@
 }
 
 // static
 void DesktopSessionProxyTraits::Destruct(
     const DesktopSessionProxy* desktop_session_proxy) {
   desktop_session_proxy->caller_task_runner_->DeleteSoon(FROM_HERE,
                                                          desktop_session_proxy);
 }
 
 }  // namespace remoting