events: Add missing #define guard in null_event_targeter.h

ui/events/null_event_targeter.h

 // Copyright 2015 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.
 
+#ifndef UI_EVENTS_NULL_EVENT_TARGETER_H_
+#define UI_EVENTS_NULL_EVENT_TARGETER_H_
+
 #include "base/compiler_specific.h"
 #include "base/macros.h"
 #include "ui/events/event_targeter.h"
 #include "ui/events/events_export.h"
 
 namespace ui {
 
 // NullEventTargeter can be installed on a root window to prevent it from
 // dispatching events such as during shutdown.
 class EVENTS_EXPORT NullEventTargeter : public EventTargeter {
  public:
   NullEventTargeter();
   ~NullEventTargeter() override;
 
   // EventTargeter:
   EventTarget* FindTargetForEvent(EventTarget* root, Event* event) override;
   EventTarget* FindNextBestTarget(EventTarget* previous_target,
                                   Event* event) override;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(NullEventTargeter);
 };
 
 }  // namespace ui
+
+#endif  // UI_EVENTS_NULL_EVENT_TARGETER_H_