Make Surface::GetEligibleFrame const

cc/surfaces/surface.h

 // Copyright 2014 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_SURFACES_SURFACE_H_
 #define CC_SURFACES_SURFACE_H_
 
 #include "base/containers/hash_tables.h"
 #include "base/macros.h"
 #include "base/memory/scoped_ptr.h"
@@ skipping 10 matching lines @@
 class CC_SURFACES_EXPORT Surface {
  public:
   Surface(SurfaceId id, const gfx::Size& size, SurfaceFactory* factory);
   ~Surface();
 
   const gfx::Size& size() const { return size_; }
   SurfaceId surface_id() const { return surface_id_; }
 
   void QueueFrame(scoped_ptr<CompositorFrame> frame);
   // Returns the most recent frame that is eligible to be rendered.
-  CompositorFrame* GetEligibleFrame();
+  const CompositorFrame* GetEligibleFrame();
 
   SurfaceFactory* factory() { return factory_; }
 
  private:
   SurfaceId surface_id_;
   gfx::Size size_;
   SurfaceFactory* factory_;
   // TODO(jamesr): Support multiple frames in flight.
   scoped_ptr<CompositorFrame> current_frame_;
 
   DISALLOW_COPY_AND_ASSIGN(Surface);
 };
 
 }  // namespace cc
 
 #endif  // CC_SURFACES_SURFACE_H_