Ensure alpha channel in backbuffer is correct with DirectComposition.

ui/gl/gl_surface.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_H_
 #define UI_GL_GL_SURFACE_H_
 
 #include <string>
 
 #include "base/callback.h"
@@ skipping 165 matching lines @@
                                unsigned edge_aa_mask,
                                const RectF& bounds_rect,
                                bool is_clipped,
                                const RectF& clip_rect,
                                const Transform& transform);
 
   virtual bool IsSurfaceless() const;
 
   virtual bool FlipsVertically() const;
 
+  // Returns true if SwapBuffers or PostSubBuffers causes a flip, such that
+  // the next buffer may be 2 frames old.
+  virtual bool BuffersFlipped() const;
+
   // Create a GL surface that renders directly to a view.
   static scoped_refptr<GLSurface> CreateViewGLSurface(
       gfx::AcceleratedWidget window);
 
 #if defined(USE_OZONE)
   // Create a GL surface that renders directly into a window with surfaceless
   // semantics - there is no default framebuffer and the primary surface must
   // be presented as an overlay. If surfaceless mode is not supported or
   // enabled it will return a null pointer.
   static scoped_refptr<GLSurface> CreateSurfacelessViewGLSurface(
@@ skipping 67 matching lines @@
   void* GetConfig() override;
   unsigned GetFormat() override;
   VSyncProvider* GetVSyncProvider() override;
   bool ScheduleOverlayPlane(int z_order,
                             OverlayTransform transform,
                             gl::GLImage* image,
                             const Rect& bounds_rect,
                             const RectF& crop_rect) override;
   bool IsSurfaceless() const override;
   bool FlipsVertically() const override;
+  bool BuffersFlipped() const override;
 
   GLSurface* surface() const { return surface_.get(); }
 
  protected:
   ~GLSurfaceAdapter() override;
 
  private:
   scoped_refptr<GLSurface> surface_;
 
   DISALLOW_COPY_AND_ASSIGN(GLSurfaceAdapter);
 };
 
 }  // namespace gfx
 
 #endif  // UI_GL_GL_SURFACE_H_