Do not give GPU memory to backgrounded compositors.

cc/output/delegating_renderer.h

 // Copyright 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 CC_OUTPUT_DELEGATING_RENDERER_H_
 #define CC_OUTPUT_DELEGATING_RENDERER_H_
 
 #include "base/memory/scoped_ptr.h"
 #include "cc/base/cc_export.h"
 #include "cc/output/compositor_frame.h"
@@ skipping 31 matching lines @@
   virtual void GetFramebufferPixels(void* pixels, gfx::Rect rect) OVERRIDE;
 
   virtual bool IsContextLost() OVERRIDE;
 
   virtual void SetVisible(bool visible) OVERRIDE;
 
   virtual void SendManagedMemoryStats(size_t bytes_visible,
                                       size_t bytes_visible_and_nearby,
                                       size_t bytes_allocated) OVERRIDE;
 
-  virtual void SetDiscardBackBufferWhenNotVisible(bool discard) OVERRIDE;
-
  private:
   DelegatingRenderer(RendererClient* client,
                      const LayerTreeSettings* settings,
                      OutputSurface* output_surface,
                      ResourceProvider* resource_provider);
   bool Initialize();
 
   OutputSurface* output_surface_;
   ResourceProvider* resource_provider_;
   RendererCapabilities capabilities_;
   CompositorFrame frame_for_swap_buffers_;
   bool visible_;
 
   DISALLOW_COPY_AND_ASSIGN(DelegatingRenderer);
 };
 
 }  // namespace cc
 
 #endif  // CC_OUTPUT_DELEGATING_RENDERER_H_