Add presubmit check for scoped_ptr usage.

ui/events/event_dispatcher_unittest.cc

 // Copyright (c) 2012 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/event_dispatcher.h"
 
 #include "testing/gtest/include/gtest/gtest.h"
 #include "ui/events/event.h"
 #include "ui/events/event_dispatcher.h"
 #include "ui/events/event_target.h"
@@ skipping 25 matching lines @@
     valid_ = true;
   }
 
  private:
   // Overridden from EventTarget:
   bool CanAcceptEvent(const ui::Event& event) override { return true; }
 
   EventTarget* GetParentTarget() override { return parent_; }
 
   scoped_ptr<EventTargetIterator> GetChildIterator() const override {
-    return scoped_ptr<EventTargetIterator>();
+    return nullptr;
   }
 
   EventTargeter* GetEventTargeter() override { return NULL; }
 
   TestTarget* parent_;
   std::vector<int> handler_list_;
   bool valid_;
 
   DISALLOW_COPY_AND_ASSIGN(TestTarget);
 };
@@ skipping 535 matching lines @@
     NonCancelableEvent event;
     EventDispatchDetails details = dispatcher->ProcessEvent(&target, &event);
     EXPECT_TRUE(details.dispatcher_destroyed);
     EXPECT_EQ(2U, target.handler_list().size());
     EXPECT_EQ(1, target.handler_list()[0]);
     EXPECT_EQ(2, target.handler_list()[1]);
   }
 }
 
 }  // namespace ui