zero-copy: Clarify to allocate/destroy GpuMemoryBuffer on any thread and use it on the main thread o

ui/gl/gl_image_surface_texture.h

 // Copyright 2014 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_SURFACE_TEXTURE_H_
 #define UI_GL_GL_IMAGE_SURFACE_TEXTURE_H_
 
 #include "base/memory/ref_counted.h"
+#include "base/threading/thread_checker.h"
 #include "ui/gl/gl_bindings.h"
 #include "ui/gl/gl_image.h"
 
 namespace gfx {
 class SurfaceTexture;
 
 class GL_EXPORT GLImageSurfaceTexture : public GLImage {
  public:
   explicit GLImageSurfaceTexture(const gfx::Size& size);
 
@@ skipping 15 matching lines @@
                             const Rect& bounds_rect,
                             const RectF& crop_rect) override;
 
  protected:
   ~GLImageSurfaceTexture() override;
 
  private:
   scoped_refptr<SurfaceTexture> surface_texture_;
   const gfx::Size size_;
   GLint texture_id_;
+  base::ThreadChecker thread_checker_;
 
   DISALLOW_COPY_AND_ASSIGN(GLImageSurfaceTexture);
 };
 
 }  // namespace gfx
 
 #endif  // UI_GL_GL_IMAGE_SURFACE_TEXTURE_H_