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

ui/ozone/public/surface_factory_ozone.h

Index: ui/ozone/public/surface_factory_ozone.h
diff --git a/ui/ozone/public/surface_factory_ozone.h b/ui/ozone/public/surface_factory_ozone.h
index 4e223bf1fc8b8abc9252978c29a53e7aaf5179ff..da524ec84bbbfb26f724f3b9b491d3053425484a 100644
--- a/ui/ozone/public/surface_factory_ozone.h
+++ b/ui/ozone/public/surface_factory_ozone.h
@@ -78,7 +78,6 @@ class OZONE_BASE_EXPORT SurfaceFactoryOzone {
   typedef base::Callback<void(GLGetProcAddressProc)>
       SetGLGetProcAddressProcCallback;
 
-  SurfaceFactoryOzone();
   virtual ~SurfaceFactoryOzone();
 
   // Returns the singleton instance.
@@ -133,6 +132,7 @@ class OZONE_BASE_EXPORT SurfaceFactoryOzone {
   // Create a single native buffer to be used for overlay planes or zero copy
   // for |widget| representing a particular display controller or default
   // display controller for kNullAcceleratedWidget.
+  // It can be called on any thread.
   virtual scoped_refptr<NativePixmap> CreateNativePixmap(
       gfx::AcceleratedWidget widget,
       gfx::Size size,
@@ -166,8 +166,11 @@ class OZONE_BASE_EXPORT SurfaceFactoryOzone {
   // such as MAP for zero copy or SCANOUT for display controller.
   virtual bool CanCreateNativePixmap(BufferUsage usage);
 
+ protected:
+  SurfaceFactoryOzone();
+
  private:
-  static SurfaceFactoryOzone* impl_;  // not owned
+  DISALLOW_COPY_AND_ASSIGN(SurfaceFactoryOzone);
 };
 
 }  // namespace ui