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 128b40a608e5eb4745d36691ce2448d0bda04315..b019f587efcba19d8f779eca14881f0c18724107 100644
--- a/third_party/WebKit/Source/core/frame/DOMWindow.h
+++ b/third_party/WebKit/Source/core/frame/DOMWindow.h
@@ -203,7 +203,7 @@ 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
@@ -268,6 +268,7 @@ class CORE_EXPORT DOMWindow : public EventTargetWithInlineData,
 
  private:
   mutable Member<Location> m_location;
+  Member<External> m_external;
 };
 
 }  // namespace blink