Ensure alpha channel in backbuffer is correct with DirectComposition.

ui/gl/gl_surface_egl.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_EGL_H_
 #define UI_GL_GL_SURFACE_EGL_H_
 
 #if defined(OS_WIN)
 #include <windows.h>
 #endif
@@ skipping 84 matching lines @@
   gfx::SwapResult PostSubBuffer(int x, int y, int width, int height) override;
   bool SupportsCommitOverlayPlanes() override;
   gfx::SwapResult CommitOverlayPlanes() override;
   VSyncProvider* GetVSyncProvider() override;
   bool ScheduleOverlayPlane(int z_order,
                             OverlayTransform transform,
                             gl::GLImage* image,
                             const Rect& bounds_rect,
                             const RectF& crop_rect) override;
   bool FlipsVertically() const override;
+  bool BuffersFlipped() const override;
 
   // Create a NativeViewGLSurfaceEGL with an externally provided VSyncProvider.
   // Takes ownership of the VSyncProvider.
   virtual bool Initialize(scoped_ptr<VSyncProvider> sync_provider);
 
   // Takes care of the platform dependant bits, of any, for creating the window.
   virtual bool InitializeNativeWindow();
 
  protected:
   ~NativeViewGLSurfaceEGL() override;
@@ skipping 86 matching lines @@
   ~SurfacelessEGL() override;
 
  private:
   gfx::Size size_;
   DISALLOW_COPY_AND_ASSIGN(SurfacelessEGL);
 };
 
 }  // namespace gfx
 
 #endif  // UI_GL_GL_SURFACE_EGL_H_