Discard compositor frames from unloaded web content

content/renderer/render_widget.h

Index: content/renderer/render_widget.h
diff --git a/content/renderer/render_widget.h b/content/renderer/render_widget.h
index e808bcc2e254bca5dbfc8b95f3bd051bcf25b37f..e43cd4db32d26c5e6869ce010d090fbb315c0f4b 100644
--- a/content/renderer/render_widget.h
+++ b/content/renderer/render_widget.h
@@ -414,6 +414,9 @@ class CONTENT_EXPORT RenderWidget
   virtual void TransferActiveWheelFlingAnimation(
       const blink::WebActiveWheelFlingParameters& params) {}
 
+  uint32_t GetContentSourceId();
+  void IncrementContentSourceId();
+
  protected:
   // Friend RefCounted so that the dtor can be non-public. Using this class
   // without ref-counting is an error.
@@ -847,6 +850,19 @@ class CONTENT_EXPORT RenderWidget
   bool time_to_first_active_paint_recorded_;
   base::TimeTicks was_shown_time_;
 
+  // This is initialized to zero and is incremented on each non-same-page
+  // navigation commit by RenderFrameImpl. At that time it is sent to the
+  // compositor so that it can tag compositor frames, and RenderFrameImpl is
+  // responsible for sending it to the browser process to be used to match
+  // each compositor frame to the most recent page navigation before it was
+  // generated.
+  // This only applies to main frames, and is not touched for subframe
+  // RenderWidgets, where there is no concern around displaying unloaded
+  // content.
+  // TODO(kenrb, fsamuel): This should be removed when SurfaceIDs can be used
+  // to replace it. See https://crbug.com/695579.
+  uint32_t current_content_source_id_;
+
   base::WeakPtrFactory<RenderWidget> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(RenderWidget);