Add a missing header guard for DOMWindow.h

Source/core/frame/DOMWindow.h

 // Copyright 2014 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 DOMWindow_h
+#define DOMWindow_h
+
 namespace blink {
 
 class ScriptWrappable;
 
 class DOMWindow {
 public:
     // Ideally, DOMWindow would be a ScriptWrappable. However, LocalDOMWindow
     // already has a ScriptWrappable subclass via EventTargetWithInlineData. As
     // a result, making DOMWindow a ScriptWrappable would force callers of
     // ScriptWrappable methods on DOMWindow methods to disambiguate which base
     // class to access the methods through. To avoid this trouble, DOMWindow
     // exposes an interface method to convert the DOMWindow to a ScriptWrappable
     // instead.
     virtual ScriptWrappable* toScriptWrappable() = 0;
 };
 
 } // namespace blink
+
+#endif // DOMWindow_h