Add support for EXT_image_flush_external extension

ui/gl/gl_surface_overlay.h

 // 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.
 
 #ifndef UI_GL_GL_SURFACE_OVERLAY_H_
 #define UI_GL_GL_SURFACE_OVERLAY_H_
 
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "ui/gfx/geometry/rect.h"
@@ skipping 13 matching lines @@
                    GLImage* image,
                    const gfx::Rect& bounds_rect,
                    const gfx::RectF& crop_rect);
   GLSurfaceOverlay(const GLSurfaceOverlay& other);
   ~GLSurfaceOverlay();
 
   // Schedule the image as an overlay plane to be shown at swap time for
   // |widget|.
   bool ScheduleOverlayPlane(gfx::AcceleratedWidget widget) const;
 
+  void Flush() const;
+
  private:
   int z_order_;
   gfx::OverlayTransform transform_;
   scoped_refptr<GLImage> image_;
   gfx::Rect bounds_rect_;
   gfx::RectF crop_rect_;
 };
 
 }  // namespace gl
 
 #endif  // UI_GL_GL_SURFACE_OVERLAY_H_