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

Index: ui/ozone/public/surface_factory_ozone.cc
diff --git a/ui/ozone/public/surface_factory_ozone.cc b/ui/ozone/public/surface_factory_ozone.cc
index 1c57038d32452d9eecc4bea34178b9b65e8265af..836386878ef8a7b23cc0c5889f88bdcd3197f930 100644
--- a/ui/ozone/public/surface_factory_ozone.cc
+++ b/ui/ozone/public/surface_factory_ozone.cc
@@ -22,12 +22,14 @@ SurfaceFactoryOzone::SurfaceFactoryOzone() {
 }
 
 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_;
 }