Add presubmit check for scoped_ptr usage.

ui/events/test/test_event_target.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/events/test/test_event_target.h"
 
 #include "ui/events/event.h"
 #include "ui/events/event_target_iterator.h"
 #include "ui/events/event_targeter.h"
 
@@ skipping 19 matching lines @@
 
 scoped_ptr<TestEventTarget> TestEventTarget::RemoveChild(TestEventTarget *c) {
   ScopedVector<TestEventTarget>::iterator iter = std::find(children_.begin(),
                                                            children_.end(),
                                                            c);
   if (iter != children_.end()) {
     children_.weak_erase(iter);
     c->set_parent(NULL);
     return scoped_ptr<TestEventTarget>(c);
   }
-  return scoped_ptr<TestEventTarget>();
+  return nullptr;
 }
 
 void TestEventTarget::SetEventTargeter(scoped_ptr<EventTargeter> targeter) {
   targeter_ = targeter.Pass();
 }
 
 bool TestEventTarget::DidReceiveEvent(ui::EventType type) const {
   return received_.count(type) > 0;
 }
 
@@ skipping 34 matching lines @@
 // TestEventTarget, private
 
 bool TestEventTarget::Contains(TestEventTarget* target) const {
   while (target && target != this)
     target = target->parent();
   return target == this;
 }
 
 }  // namespace test
 }  // namespace ui