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

Source/core/frame/DOMWindow.h

Index: Source/core/frame/DOMWindow.h
diff --git a/Source/core/frame/DOMWindow.h b/Source/core/frame/DOMWindow.h
new file mode 100644
index 0000000000000000000000000000000000000000..38f2a31aa2e8db7f95f088c12eac3b8caa3b5613
--- /dev/null
+++ b/Source/core/frame/DOMWindow.h
@@ -0,0 +1,21 @@
+// 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