Ensure alpha channel in backbuffer is correct with DirectComposition.

ui/gl/gl_surface_stub.h

Index: ui/gl/gl_surface_stub.h
diff --git a/ui/gl/gl_surface_stub.h b/ui/gl/gl_surface_stub.h
index bf21573e1d50dfd70ed9a2eebc14110d95c09af7..9115b7c1dcd0f76b7c175669337524ead28a1f3f 100644
--- a/ui/gl/gl_surface_stub.h
+++ b/ui/gl/gl_surface_stub.h
@@ -13,19 +13,25 @@ namespace gfx {
 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