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

ui/ozone/public/surface_factory_ozone.cc

 // Copyright (c) 2013 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.
 
 #include "ui/ozone/public/surface_factory_ozone.h"
 
 #include <stdlib.h>
 
 #include "base/command_line.h"
 #include "ui/ozone/public/native_pixmap.h"
 #include "ui/ozone/public/surface_ozone_canvas.h"
 #include "ui/ozone/public/surface_ozone_egl.h"
 
 namespace ui {
 
 // static
 SurfaceFactoryOzone* SurfaceFactoryOzone::impl_ = NULL;
 
 SurfaceFactoryOzone::SurfaceFactoryOzone() {
   DCHECK(!impl_) << "There should only be a single SurfaceFactoryOzone.";
   impl_ = this;
 }
 
 SurfaceFactoryOzone::~SurfaceFactoryOzone() {
+  DCHECK(thread_checker_.CalledOnValidThread());
   DCHECK_EQ(impl_, this);
-  impl_ = NULL;
+  impl_ = nullptr;
 }
 
 SurfaceFactoryOzone* SurfaceFactoryOzone::GetInstance() {
   DCHECK(impl_) << "No SurfaceFactoryOzone implementation set.";
+  DCHECK(impl_->thread_checker_.CalledOnValidThread());
   return impl_;
 }
 
 intptr_t SurfaceFactoryOzone::GetNativeDisplay() {
   return 0;
 }
 
 int SurfaceFactoryOzone::GetDrmFd() {
   return -1;
 }
@@ skipping 47 matching lines @@
 
 bool SurfaceFactoryOzone::CanShowPrimaryPlaneAsOverlay() {
   return false;
 }
 
 bool SurfaceFactoryOzone::CanCreateNativePixmap(BufferUsage usage) {
   return false;
 }
 
 }  // namespace ui