Add support for EXT_image_flush_external extension

gpu/command_buffer/service/gl_stream_texture_image.h

 // Copyright 2016 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 GPU_COMMAND_BUFFER_SERVICE_GL_STREAM_TEXTURE_IMAGE_H_
 #define GPU_COMMAND_BUFFER_SERVICE_GL_STREAM_TEXTURE_IMAGE_H_
 
 #include "gpu/gpu_export.h"
 #include "ui/gl/gl_image.h"
 
@@ skipping 13 matching lines @@
   virtual void GetTextureMatrix(float matrix[16]) = 0;
 
   // Copy the texture matrix for this image into |matrix|, returning a matrix
   // for which UV=(0,0) corresponds to the top left of corner of the image,
   // which is what Chromium generally expects.
   void GetFlippedTextureMatrix(float matrix[16]) {
     GetTextureMatrix(matrix);
     matrix[13] += matrix[5];
     matrix[5] = -matrix[5];
   }
+
+  void Flush() override {}
+
  protected:
   ~GLStreamTextureImage() override {}
 
  private:
   DISALLOW_COPY_AND_ASSIGN(GLStreamTextureImage);
 };
 
 }  // namespace gles2
 }  // namespace gpu
 
 #endif  // GPU_COMMAND_BUFFER_SERVICE_GL_STREAM_TEXTURE_IMAGE_H_