Add a basic DOMWindow base class and use it in WindowProxy.

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.
+
+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