Delete background tab IOSurfaces on Mac.

content/browser/renderer_host/render_widget_host_impl.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CONTENT_BROWSER_RENDERER_HOST_RENDER_WIDGET_HOST_IMPL_H_
 #define CONTENT_BROWSER_RENDERER_HOST_RENDER_WIDGET_HOST_IMPL_H_
 #pragma once
 
 #include <deque>
 #include <string>
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   bool is_accelerated_compositing_active() {
     return is_accelerated_compositing_active_;
   }
 
   // Notifies the RenderWidgetHost that the View was destroyed.
   void ViewDestroyed();
 
   // 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
   // (for example, if we don't currently have a RenderWidgetHostView.)
   BackingStore* AllocBackingStore(const gfx::Size& size);
 
   // When a backing store does asynchronous painting, it will call this function
   // when it is done with the DIB. We will then forward a message to the
   // renderer to send another paint.
   void DonePaintingToBackingStore();
@@ skipping 510 matching lines @@
   base::WeakPtrFactory<RenderWidgetHostImpl> weak_factory_;
 
   scoped_ptr<TapSuppressionController> tap_suppression_controller_;
 
   DISALLOW_COPY_AND_ASSIGN(RenderWidgetHostImpl);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_RENDERER_HOST_RENDER_WIDGET_HOST_IMPL_H_