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..615395fc756599549e95e911e56cb05b4ab9bafb 100644
--- a/ui/ozone/public/surface_factory_ozone.cc
+++ b/ui/ozone/public/surface_factory_ozone.cc
@@ -7,28 +7,33 @@
 #include <stdlib.h>
 
 #include "base/command_line.h"
+#include "base/lazy_instance.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;
+namespace {
+
+base::LazyInstance<SurfaceFactoryOzone*>::Leaky g_impl =
+    LAZY_INSTANCE_INITIALIZER;
+
+}  // namespace
 
 SurfaceFactoryOzone::SurfaceFactoryOzone() {
-  DCHECK(!impl_) << "There should only be a single SurfaceFactoryOzone.";
-  impl_ = this;
+  DCHECK(!g_impl.Get()) << "There should only be a single SurfaceFactoryOzone.";
+  *g_impl.Pointer() = this;
 }
 
 SurfaceFactoryOzone::~SurfaceFactoryOzone() {
-  DCHECK_EQ(impl_, this);
-  impl_ = NULL;
+  DCHECK_EQ(g_impl.Get(), this);
+  *g_impl.Pointer() = nullptr;
 }
 
 SurfaceFactoryOzone* SurfaceFactoryOzone::GetInstance() {
-  DCHECK(impl_) << "No SurfaceFactoryOzone implementation set.";
-  return impl_;
+  DCHECK(g_impl.Get()) << "No SurfaceFactoryOzone implementation set.";
+  return g_impl.Get();
 }
 
 intptr_t SurfaceFactoryOzone::GetNativeDisplay() {
@@ -65,7 +70,7 @@ const int32* SurfaceFactoryOzone::GetEGLSurfaceProperties(
 
 ui::OverlayCandidatesOzone* SurfaceFactoryOzone::GetOverlayCandidates(
     gfx::AcceleratedWidget w) {
-  return NULL;
+  return nullptr;
 }
 
 scoped_refptr<ui::NativePixmap> SurfaceFactoryOzone::CreateNativePixmap(
@@ -73,7 +78,7 @@ scoped_refptr<ui::NativePixmap> SurfaceFactoryOzone::CreateNativePixmap(
     gfx::Size size,
     BufferFormat format,
     BufferUsage usage) {
-  return NULL;
+  return nullptr;
 }
 
 bool SurfaceFactoryOzone::ScheduleOverlayPlane(