Use observer pattern instead of sniffing SwapCompositorFrame IPC

content/browser/renderer_host/render_widget_host_impl.h

 // 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.
 
 #ifndef CONTENT_BROWSER_RENDERER_HOST_RENDER_WIDGET_HOST_IMPL_H_
 #define CONTENT_BROWSER_RENDERER_HOST_RENDER_WIDGET_HOST_IMPL_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 559 matching lines @@
   // unnecessary IPCs to RenderWidget (e.g., asking for monitor updates while
   // we are already receiving updates), when
   // |monitoring_composition_info_| == |monitor_updates| no IPC is sent to the
   // renderer unless it is for an immediate request.
   void RequestCompositionUpdates(bool immediate_request, bool monitor_updates);
 
   // Submits the frame received from RenderWidget.
   void SubmitCompositorFrame(const cc::LocalSurfaceId& local_surface_id,
                              cc::CompositorFrame frame);
 
+  const cc::CompositorFrameMetadata& last_frame_metadata() {
+    return last_frame_metadata_;
+  }
+
  protected:
   // ---------------------------------------------------------------------------
   // The following method is overridden by RenderViewHost to send upwards to
   // its delegate.
 
   // Callback for notification that we failed to receive any rendered graphics
   // from a newly loaded page. Used for testing.
   virtual void NotifyNewContentRenderingTimeoutForTesting() {}
 
   // ---------------------------------------------------------------------------
@@ skipping 351 matching lines @@
   // factor change, a new LocalSurfaceId must be created.
   cc::LocalSurfaceId last_local_surface_id_;
   gfx::Size last_frame_size_;
   float last_device_scale_factor_;
 
   // Each instance of RendererCompositorFrameSink has an ID that we keep track
   // of so we can tell when a new instance has been created for the purpose of
   // not returning stale resources.
   uint32_t last_compositor_frame_sink_id_ = 0;
 
+  cc::CompositorFrameMetadata last_frame_metadata_;
+
   base::WeakPtrFactory<RenderWidgetHostImpl> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(RenderWidgetHostImpl);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_RENDERER_HOST_RENDER_WIDGET_HOST_IMPL_H_