Add support for EXT_image_flush_external extension

ui/gl/gl_image.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_H_
 #define UI_GL_GL_IMAGE_H_
 
 #include <stdint.h>
 
 #include <string>
@@ skipping 74 matching lines @@
   // removed. https://crbug.com/581777#c36
   virtual bool EmulatingRGB() const;
 
   // An identifier for subclasses. Necessary for safe downcasting.
   enum class Type {
     NONE,
     IOSURFACE
   };
   virtual Type GetType() const;
 
+  // Flush any preceding rendering for the image.
+  virtual void Flush() = 0;

[reveman, 2016/07/11 18:59:06]

nit: EmulatingRGB and GetType are temporary workarounds but I think this Flush
function is part of the core API. Please move it below ScheduleOverlayPlane and
maintain the sorting order where implemented.

+
  protected:
   virtual ~GLImage() {}
 
  private:
   friend class base::RefCounted<GLImage>;
 
   DISALLOW_COPY_AND_ASSIGN(GLImage);
 };
 
 }  // namespace gl
 
 #endif  // UI_GL_GL_IMAGE_H_