Add support for EXT_image_flush_external extension

ui/gl/gl_image_egl.h

 // Copyright (c) 2013 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_IMAGE_EGL_H_
 #define UI_GL_GL_IMAGE_EGL_H_
 
 #include "base/macros.h"
 #include "base/threading/thread_checker.h"
 #include "ui/gl/gl_bindings.h"
@@ skipping 16 matching lines @@
   void ReleaseTexImage(unsigned target) override {}
   bool CopyTexImage(unsigned target) override;
   bool CopyTexSubImage(unsigned target,
                        const gfx::Point& offset,
                        const gfx::Rect& rect) override;
   bool ScheduleOverlayPlane(gfx::AcceleratedWidget widget,
                             int z_order,
                             gfx::OverlayTransform transform,
                             const gfx::Rect& bounds_rect,
                             const gfx::RectF& crop_rect) override;
+  void Flush() override {}
 
  protected:
   ~GLImageEGL() override;
 
   EGLImageKHR egl_image_;
   const gfx::Size size_;
   base::ThreadChecker thread_checker_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(GLImageEGL);
 };
 
 }  // namespace gl
 
 #endif  // UI_GL_GL_IMAGE_EGL_H_