Add presubmit check for scoped_ptr usage.

ui/wm/core/nested_accelerator_dispatcher_linux.cc

Index: ui/wm/core/nested_accelerator_dispatcher_linux.cc
diff --git a/ui/wm/core/nested_accelerator_dispatcher_linux.cc b/ui/wm/core/nested_accelerator_dispatcher_linux.cc
index 3016fc031b4c415a9f867db0ca26313d523c2b6c..a5ce30a381ab174e29a953039a4a613578c2152d 100644
--- a/ui/wm/core/nested_accelerator_dispatcher_linux.cc
+++ b/ui/wm/core/nested_accelerator_dispatcher_linux.cc
@@ -38,8 +38,7 @@ bool IsKeyEvent(const XEvent* xev) {
 scoped_ptr<ui::ScopedEventDispatcher> OverrideDispatcher(
     ui::PlatformEventDispatcher* dispatcher) {
   ui::PlatformEventSource* source = ui::PlatformEventSource::GetInstance();
-  return source ? source->OverrideDispatcher(dispatcher)
-                : scoped_ptr<ui::ScopedEventDispatcher>();
+  return source ? source->OverrideDispatcher(dispatcher) : nullptr;
 }
 
 }  // namespace