Ensure alpha channel in backbuffer is correct with DirectComposition.

ui/gl/gl_surface_stub.h

 // 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.
 
 #ifndef UI_GL_GL_SURFACE_STUB_H_
 #define UI_GL_GL_SURFACE_STUB_H_
 
 #include "ui/gl/gl_surface.h"
 
 namespace gfx {
 
 // A GLSurface that does nothing for unit tests.
 class GL_EXPORT GLSurfaceStub : public GLSurface {
  public:
   void SetSize(const gfx::Size& size) { size_ = size; }
+  void set_buffers_flipped(bool flipped) { buffers_flipped_ = flipped; }
 
   // Implement GLSurface.
   void Destroy() override;
+  bool Resize(const gfx::Size& size,
+              float scale_factor,
+              bool has_alpha) override;
   bool IsOffscreen() override;
   gfx::SwapResult SwapBuffers() override;
   gfx::Size GetSize() override;
   void* GetHandle() override;
+  bool BuffersFlipped() const override;
 
  protected:
   ~GLSurfaceStub() override;
 
  private:
   gfx::Size size_;
+  bool buffers_flipped_ = false;
 };
 
 }  // namespace gfx
 
 #endif  // UI_GL_GL_SURFACE_STUB_H_