Remove code only used by Mac single threaded compositing.

content/port/browser/render_widget_host_view_port.h

Index: content/port/browser/render_widget_host_view_port.h
diff --git a/content/port/browser/render_widget_host_view_port.h b/content/port/browser/render_widget_host_view_port.h
index a668af677451e6d31b75c2a38193cf5bf7c8e59f..54d6069f990b471c572f3f7261dc972c0b965895 100644
--- a/content/port/browser/render_widget_host_view_port.h
+++ b/content/port/browser/render_widget_host_view_port.h
@@ -318,9 +318,6 @@ class CONTENT_EXPORT RenderWidgetHostViewPort : public RenderWidgetHostView,
   virtual void DidReceiveRendererFrame() = 0;
 
 #if defined(OS_MACOSX)
-  // Called just before GetBackingStore blocks for an updated frame.
-  virtual void AboutToWaitForBackingStoreMsg() = 0;
-
   // Does any event handling necessary for plugin IME; should be called after
   // the plugin has already had a chance to process the event. If plugin IME is
   // not enabled, this is a no-op, so it is always safe to call.