Ensure alpha channel in backbuffer is correct with DirectComposition.

ui/gl/gl_surface_stub.cc

 // Copyright (c) 2012 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 "ui/gl/gl_surface_stub.h"
 
 namespace gfx {
 
 void GLSurfaceStub::Destroy() {
 }
 
+bool GLSurfaceStub::Resize(const gfx::Size& size,
+                           float scale_factor,
+                           bool has_alpha) {
+  return true;
+}
+
 bool GLSurfaceStub::IsOffscreen() {
   return false;
 }
 
 gfx::SwapResult GLSurfaceStub::SwapBuffers() {
   return gfx::SwapResult::SWAP_ACK;
 }
 
 gfx::Size GLSurfaceStub::GetSize() {
   return size_;
 }
 
 void* GLSurfaceStub::GetHandle() {
   return NULL;
 }
 
+bool GLSurfaceStub::BuffersFlipped() const {
+  return buffers_flipped_;
+}
+
 GLSurfaceStub::~GLSurfaceStub() {}
 
 }  // namespace gfx