cc: Make OutputSurface::SwapBuffers pure virtual

cc/test/pixel_test_output_surface.h

 // Copyright 2013 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_TEST_PIXEL_TEST_OUTPUT_SURFACE_H_
 #define CC_TEST_PIXEL_TEST_OUTPUT_SURFACE_H_
 
 #include "cc/output/output_surface.h"
 
 namespace cc {
 
 class PixelTestOutputSurface : public OutputSurface {
  public:
   explicit PixelTestOutputSurface(
       scoped_refptr<ContextProvider> context_provider);
   explicit PixelTestOutputSurface(
       scoped_ptr<SoftwareOutputDevice> software_device);
 
   virtual void Reshape(const gfx::Size& size, float scale_factor) override;
   virtual bool HasExternalStencilTest() const override;
+  virtual void SwapBuffers(CompositorFrame* frame) override;
 
   void set_surface_expansion_size(const gfx::Size& surface_expansion_size) {
     surface_expansion_size_ = surface_expansion_size;
   }
   void set_has_external_stencil_test(bool has_test) {
     external_stencil_test_ = has_test;
   }
 
  private:
   gfx::Size surface_expansion_size_;
   bool external_stencil_test_;
 };
 
 }  // namespace cc
 
 #endif  // CC_TEST_PIXEL_TEST_OUTPUT_SURFACE_H_