Use observer pattern instead of sniffing SwapCompositorFrame IPC

content/browser/renderer_host/render_widget_host_impl.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 "content/browser/renderer_host/render_widget_host_impl.h"
 
 #include <math.h>
 
 #include <set>
 #include <tuple>
@@ skipping 2591 matching lines @@
       (frame_size != last_frame_size_ ||
        device_scale_factor != last_device_scale_factor_)) {
     DLOG(ERROR) << "Renderer submitted frame of wrong size to its surface."
                 << " Expected: size=" << last_frame_size_.ToString()
                 << ",scale=" << last_device_scale_factor_
                 << " Received: size=" << frame_size.ToString()
                 << ",scale=" << device_scale_factor;
     return;
   }
 
+  last_frame_metadata_ = frame.metadata.Clone();

[piman, 2017/04/04 17:34:46]

Could we move this to after l.2636? 2 reasons:
1- we don't want to update the metadata if we reject the bad message
2- we modify frame.metadata below, I think we want last_frame_metadata_ to be
consistent with that.

[Saman Sami, 2017/04/04 17:49:54]

Good point.

+  if (delegate_)
+    delegate_->DidReceiveCompositorFrame();

[piman, 2017/04/04 17:34:46]

Should this move to the end of the function, or at least after l.2658? For
consistent state it may be better to signal the observers after the view was
updated.

[Saman Sami, 2017/04/04 17:49:54]

Done.

+
   last_local_surface_id_ = local_surface_id;
   last_frame_size_ = frame_size;
   last_device_scale_factor_ = device_scale_factor;
 
   last_received_content_source_id_ = frame.metadata.content_source_id;
 
   if (frame.metadata.begin_frame_ack.sequence_number <
       cc::BeginFrameArgs::kStartingFrameNumber) {
     // Received an invalid ack, renderer misbehaved.
     bad_message::ReceivedBadMessage(
@@ skipping 33 matching lines @@
 
   // After navigation, if a frame belonging to the new page is received, stop
   // the timer that triggers clearing the graphics of the last page.
   if (last_received_content_source_id_ >= current_content_source_id_ &&
       new_content_rendering_timeout_->IsRunning()) {
     new_content_rendering_timeout_->Stop();
   }
 }
 
 }  // namespace content