Use observer pattern instead of sniffing SwapCompositorFrame IPC

content/browser/renderer_host/render_widget_host_impl.h

Index: content/browser/renderer_host/render_widget_host_impl.h
diff --git a/content/browser/renderer_host/render_widget_host_impl.h b/content/browser/renderer_host/render_widget_host_impl.h
index 8adab6b820dee2bf94c9bfbe229912ead5f63fa6..5ae6c05a92d9f5953ec525e658ad0d9404d5b6a3 100644
--- a/content/browser/renderer_host/render_widget_host_impl.h
+++ b/content/browser/renderer_host/render_widget_host_impl.h
@@ -577,6 +577,10 @@ class CONTENT_EXPORT RenderWidgetHostImpl : public RenderWidgetHost,
   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
@@ -948,6 +952,8 @@ class CONTENT_EXPORT RenderWidgetHostImpl : public RenderWidgetHost,
   // 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);