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

ui/ozone/platform/drm/drm_surface_factory.h

Index: ui/ozone/platform/drm/drm_surface_factory.h
diff --git a/ui/ozone/platform/drm/drm_surface_factory.h b/ui/ozone/platform/drm/drm_surface_factory.h
index 1dfdf1ad848e856c0ff808d63cf084ff564d285f..3baa1754065c72f2442078639253daf7a17c78ca 100644
--- a/ui/ozone/platform/drm/drm_surface_factory.h
+++ b/ui/ozone/platform/drm/drm_surface_factory.h
@@ -6,6 +6,7 @@
 #define UI_OZONE_PLATFORM_DRM_DRM_SURFACE_FACTORY_H_
 
 #include "base/memory/scoped_ptr.h"
+#include "base/threading/thread_checker.h"
 #include "ui/ozone/public/surface_factory_ozone.h"
 
 namespace ui {
@@ -30,6 +31,7 @@ class DrmSurfaceFactory : public SurfaceFactoryOzone {
 
  protected:
   ScreenManager* screen_manager_;  // Not owned.
+  base::ThreadChecker thread_checker_;
 
   DISALLOW_COPY_AND_ASSIGN(DrmSurfaceFactory);
 };