Ensure alpha channel in backbuffer is correct with DirectComposition.

ui/gl/gl_surface_egl.cc

Index: ui/gl/gl_surface_egl.cc
diff --git a/ui/gl/gl_surface_egl.cc b/ui/gl/gl_surface_egl.cc
index f748cfcc887978ae79d0a6dcbb212ffb41fb5af8..e316b2688129935215a30e61fdf599209d42e4b5 100644
--- a/ui/gl/gl_surface_egl.cc
+++ b/ui/gl/gl_surface_egl.cc
@@ -764,6 +764,10 @@ bool NativeViewGLSurfaceEGL::FlipsVertically() const {
   return flips_vertically_;
 }
 
+bool NativeViewGLSurfaceEGL::BuffersFlipped() const {
+  return g_use_direct_composition;
+}
+
 gfx::SwapResult NativeViewGLSurfaceEGL::PostSubBuffer(int x,
                                                       int y,
                                                       int width,