Add support for EXT_image_flush_external extension

ui/gl/gl_image_stub.h

 // Copyright (c) 2012 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_STUB_H_
 #define UI_GL_GL_IMAGE_STUB_H_
 
 #include <stdint.h>
 
 #include "ui/gl/gl_export.h"
@@ skipping 17 matching lines @@
                        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 OnMemoryDump(base::trace_event::ProcessMemoryDump* pmd,
                     uint64_t process_tracing_id,
                     const std::string& dump_name) override {}
+  void Flush() override {}
 
  protected:
   ~GLImageStub() override;
 };
 
 }  // namespace gl
 
 #endif  // UI_GL_GL_IMAGE_STUB_H_