Add presubmit check for scoped_ptr usage.

ui/events/event_dispatcher_unittest.cc

Index: ui/events/event_dispatcher_unittest.cc
diff --git a/ui/events/event_dispatcher_unittest.cc b/ui/events/event_dispatcher_unittest.cc
index 52becb583719a75d5cd56ab625c0a38201564ea9..95b45a3071d24fcc92481a4b61831ba0bb109cf8 100644
--- a/ui/events/event_dispatcher_unittest.cc
+++ b/ui/events/event_dispatcher_unittest.cc
@@ -43,7 +43,7 @@ class TestTarget : public EventTarget {
   EventTarget* GetParentTarget() override { return parent_; }
 
   scoped_ptr<EventTargetIterator> GetChildIterator() const override {
-    return scoped_ptr<EventTargetIterator>();
+    return nullptr;
   }
 
   EventTargeter* GetEventTargeter() override { return NULL; }