Delete background tab IOSurfaces on Mac.

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 2089b3c1a7b5faf58784fc7d7eed6f215e34170a..9fd3fd6476148f038ed097faf3bae3fc4907daf5 100644
--- a/content/browser/renderer_host/render_widget_host_impl.h
+++ b/content/browser/renderer_host/render_widget_host_impl.h
@@ -180,12 +180,15 @@ class CONTENT_EXPORT RenderWidgetHostImpl : virtual public RenderWidgetHost,
   // Indicates if the page has finished loading.
   void SetIsLoading(bool is_loading);
 
-  // Get access to the widget's backing store.  If a resize is in progress,
-  // then the current size of the backing store may be less than the size of
-  // the widget's view.  If you pass |force_create| as true, then the backing
-  // store will be created if it doesn't exist. Otherwise, NULL will be returned
-  // if the backing store doesn't already exist. It will also return NULL if the
+  // Get access to the widget's backing store matching the size of the widget's
+  // view. If you pass |force_create| as true, then GetBackingStore may block
+  // for the renderer to send a new frame. Otherwise, NULL will be returned if
+  // the backing store doesn't already exist. It will also return NULL if the
   // backing store could not be created.
+  //
+  // Mac only: NULL may also be returned if the last frame was GPU accelerated.
+  // Call GetView()->HasAcceleratedSurface to determine if the last frame was
+  // accelerated.
   BackingStore* GetBackingStore(bool force_create);
 
   // Allocate a new backing store of the given size. Returns NULL on failure