Add support for EXT_image_flush_external extension

ui/gl/gl_image_ozone_native_pixmap.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_IMAGE_OZONE_NATIVE_PIXMAP_H_
 #define UI_GL_GL_IMAGE_OZONE_NATIVE_PIXMAP_H_
 
 #include <stdint.h>
 
 #include "ui/gfx/buffer_types.h"
@@ skipping 14 matching lines @@
   void Destroy(bool have_context) override;
   bool CopyTexImage(unsigned target) 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;
 
   static unsigned GetInternalFormatForTesting(gfx::BufferFormat format);
 
  protected:
   ~GLImageOzoneNativePixmap() override;
 
  private:
   unsigned internalformat_;
   scoped_refptr<ui::NativePixmap> pixmap_;
 };
 
 }  // namespace gl
 
 #endif  // UI_GL_GL_IMAGE_OZONE_NATIVE_PIXMAP_H_