cc: Make OutputSurface::SwapBuffers pure virtual

cc/test/pixel_test_output_surface.cc

 // 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.
 
 #include "cc/test/pixel_test_output_surface.h"
 
 #include "cc/output/output_surface_client.h"
 #include "ui/gfx/transform.h"
 
 namespace cc {
@@ skipping 10 matching lines @@
                                      float scale_factor) {
   gfx::Size expanded_size(size.width() + surface_expansion_size_.width(),
                           size.height() + surface_expansion_size_.height());
   OutputSurface::Reshape(expanded_size, scale_factor);
 }
 
 bool PixelTestOutputSurface::HasExternalStencilTest() const {
   return external_stencil_test_;
 }
 
+void PixelTestOutputSurface::SwapBuffers(CompositorFrame* frame) {
+  PostSwapBuffersComplete();
+  client_->DidSwapBuffers();
+}
+
 }  // namespace cc