Discard compositor frames from unloaded web content

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 b6aa99ec6f3c4147adec40a795432f45b41385ef..11dddf13c0f1ebfe7a95f53baaa4490f5c19f65c 100644
--- a/content/browser/renderer_host/render_widget_host_impl.h
+++ b/content/browser/renderer_host/render_widget_host_impl.h
@@ -325,8 +325,10 @@ class CONTENT_EXPORT RenderWidgetHostImpl : public RenderWidgetHost,
   void StopHangMonitorTimeout();
 
   // Starts the rendering timeout, which will clear displayed graphics if
-  // a new compositor frame is not received before it expires.
-  void StartNewContentRenderingTimeout();
+  // a new compositor frame is not received before it expires. This also causes
+  // any new compositor frames received with content_source_id less than
+  // |next_source_id| to be discarded.
+  void StartNewContentRenderingTimeout(uint32_t next_source_id);
 
   // Notification that a new compositor frame has been generated following
   // a page load. This stops |new_content_rendering_timeout_|, or prevents
@@ -889,6 +891,14 @@ class CONTENT_EXPORT RenderWidgetHostImpl : public RenderWidgetHost,
   // renderer process before clearing any previously displayed content.
   base::TimeDelta new_content_rendering_delay_;
 
+  // This identifier tags compositor frames according to the page load with
+  // which they are associated, to prevent an unloaded web page from being
+  // drawn after a navigation to a new page has already committed. This is
+  // a no-op for non-top-level RenderWidgets, as that should always be zero.
+  // TODO(kenrb, fsamuel): We should use SurfaceIDs for this purpose when they
+  // are available in the renderer process. See https://crbug.com/695579.
+  uint32_t current_content_source_id_;
+
 #if defined(OS_MACOSX)
   std::unique_ptr<device::PowerSaveBlocker> power_save_blocker_;
 #endif