Make cc::SurfaceId unguessable

cc/surfaces/surface_factory_client.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_FACTORY_CLIENT_H_
 #define CC_SURFACES_SURFACE_FACTORY_CLIENT_H_
 
 #include "cc/resources/returned_resource.h"
 #include "cc/surfaces/surface_id.h"
 #include "cc/surfaces/surfaces_export.h"
 #include "ui/gfx/geometry/rect.h"
 
 namespace cc {
 
 class BeginFrameSource;
-struct SurfaceId;
+class SurfaceId;
 
 class CC_SURFACES_EXPORT SurfaceFactoryClient {
  public:
   virtual ~SurfaceFactoryClient() {}
 
   virtual void ReturnResources(const ReturnedResourceArray& resources) = 0;
 
   virtual void WillDrawSurface(SurfaceId surface_id,
                                const gfx::Rect& damage_rect) {}
 
   // This allows the SurfaceFactory to pass a BeginFrameSource to use.
   virtual void SetBeginFrameSource(BeginFrameSource* begin_frame_source) = 0;
 };
 
 }  // namespace cc
 
 #endif  // CC_SURFACES_SURFACE_FACTORY_CLIENT_H_