Make External per-window rather than a static Persistent.

third_party/WebKit/Source/core/frame/DOMWindow.h

Index: third_party/WebKit/Source/core/frame/DOMWindow.h
diff --git a/third_party/WebKit/Source/core/frame/DOMWindow.h b/third_party/WebKit/Source/core/frame/DOMWindow.h
index db61486664291fa401c291f744e4732298cbf9ad..f306e143dca2dd84e2d5122719516ee4c41710e7 100644
--- a/third_party/WebKit/Source/core/frame/DOMWindow.h
+++ b/third_party/WebKit/Source/core/frame/DOMWindow.h
@@ -207,7 +207,7 @@ class CORE_EXPORT DOMWindow : public EventTargetWithInlineData,
   // 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
@@ -267,6 +267,7 @@ class CORE_EXPORT DOMWindow : public EventTargetWithInlineData,
  private:
   Member<Frame> m_frame;
   mutable Member<Location> m_location;
+  Member<External> m_external;
 
   // Set to true when close() has been called. Needed for
   // |window.closed| determinism; having it return 'true'