Ensure alpha channel in backbuffer is correct with DirectComposition.

ui/gl/gl_surface.h

Index: ui/gl/gl_surface.h
diff --git a/ui/gl/gl_surface.h b/ui/gl/gl_surface.h
index 066435aacfa36ff20a193da5703449f0b36ded8d..56b6bcfc096e474d95dab6adcbcd0ea007867e74 100644
--- a/ui/gl/gl_surface.h
+++ b/ui/gl/gl_surface.h
@@ -183,6 +183,10 @@ class GL_EXPORT GLSurface : public base::RefCounted<GLSurface> {
 
   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);
@@ -270,6 +274,7 @@ class GL_EXPORT GLSurfaceAdapter : public GLSurface {
                             const RectF& crop_rect) override;
   bool IsSurfaceless() const override;
   bool FlipsVertically() const override;
+  bool BuffersFlipped() const override;
 
   GLSurface* surface() const { return surface_.get(); }