Add support for EXT_image_flush_external extension

ui/gl/gl_surface_overlay.cc

 // Copyright 2015 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.
 
 #include "ui/gl/gl_surface_overlay.h"
 
 #include "ui/gfx/geometry/rect.h"
 #include "ui/gfx/geometry/rect_f.h"
 #include "ui/gfx/native_widget_types.h"
 #include "ui/gfx/overlay_transform.h"
@@ skipping 15 matching lines @@
 GLSurfaceOverlay::GLSurfaceOverlay(const GLSurfaceOverlay& other) = default;
 
 GLSurfaceOverlay::~GLSurfaceOverlay() {}
 
 bool GLSurfaceOverlay::ScheduleOverlayPlane(
     gfx::AcceleratedWidget widget) const {
   return image_->ScheduleOverlayPlane(widget, z_order_, transform_,
                                       bounds_rect_, crop_rect_);
 }
 
+void GLSurfaceOverlay::Flush() const {
+  return image_->Flush();
+}
+
 }  // namespace gl