Make External per-window rather than a static Persistent.

third_party/WebKit/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
 
 #include "bindings/core/v8/Transferables.h"
 #include "core/CoreExport.h"
 #include "core/events/EventTarget.h"
@@ skipping 185 matching lines @@
   virtual int requestIdleCallback(IdleRequestCallback*,
                                   const IdleRequestOptions&) = 0;
   virtual void cancelIdleCallback(int id) = 0;
 
   // Custom elements
   virtual CustomElementRegistry* customElements(ScriptState*) const = 0;
 
   // Obsolete APIs
   void captureEvents() {}
   void releaseEvents() {}
-  External* external() const;
+  External* external();
 
   // FIXME: This handles both window[index] and window.frames[index]. However,
   // the spec exposes window.frames[index] across origins but not
   // window[index]...
   DOMWindow* anonymousIndexedGetter(uint32_t) const;
 
   void postMessage(PassRefPtr<SerializedScriptValue> message,
                    const MessagePortArray&,
                    const String& targetOrigin,
                    LocalDOMWindow* source,
@@ skipping 44 matching lines @@
 
   // Set to true when close() has been called. Needed for
   // |window.closed| determinism; having it return 'true'
   // only after the layout widget's deferred window close
   // operation has been performed, exposes (confusing)
   // implementation details to scripts.
   bool m_windowIsClosing;
 
  private:
   mutable Member<Location> m_location;
+  Member<External> m_external;
 };
 
 }  // namespace blink
 
 #endif  // DOMWindow_h