Make Surface::GetEligibleFrame const

cc/surfaces/surface.cc

 // 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.
 
 #include "cc/surfaces/surface.h"
 
 #include "cc/output/compositor_frame.h"
 #include "cc/surfaces/surface_factory.h"
 
 namespace cc {
@@ skipping 20 matching lines @@
 
   if (previous_frame) {
     ReturnedResourceArray previous_resources;
     TransferableResource::ReturnResources(
         previous_frame->delegated_frame_data->resource_list,
         &previous_resources);
     factory_->UnrefResources(previous_resources);
   }
 }
 
-CompositorFrame* Surface::GetEligibleFrame() { return current_frame_.get(); }
+const CompositorFrame* Surface::GetEligibleFrame() {
+  return current_frame_.get();
+}
 
 }  // namespace cc