Ensure alpha channel in backbuffer is correct with DirectComposition.

ui/gl/gl_surface.cc

Index: ui/gl/gl_surface.cc
diff --git a/ui/gl/gl_surface.cc b/ui/gl/gl_surface.cc
index 224953658cc8bf3e13484a96fa444cc55283054d..113142b5285bff6ce3c32e9222e5bf8839ba6f55 100644
--- a/ui/gl/gl_surface.cc
+++ b/ui/gl/gl_surface.cc
@@ -223,6 +223,10 @@ bool GLSurface::FlipsVertically() const {
   return false;
 }
 
+bool GLSurface::BuffersFlipped() const {
+  return false;
+}
+
 GLSurface* GLSurface::GetCurrent() {
   return current_surface_.Pointer()->Get();
 }
@@ -387,6 +391,10 @@ bool GLSurfaceAdapter::FlipsVertically() const {
   return surface_->FlipsVertically();
 }
 
+bool GLSurfaceAdapter::BuffersFlipped() const {
+  return surface_->BuffersFlipped();
+}
+
 GLSurfaceAdapter::~GLSurfaceAdapter() {}
 
 }  // namespace gfx