Make External per-window rather than a static Persistent.

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

Index: third_party/WebKit/Source/core/frame/DOMWindow.cpp
diff --git a/third_party/WebKit/Source/core/frame/DOMWindow.cpp b/third_party/WebKit/Source/core/frame/DOMWindow.cpp
index 7e94256eeb1bf98fc362e9e48c664d7de2d55314..9c7026cf7b5dfa345302ebcaf2f650234947d904 100644
--- a/third_party/WebKit/Source/core/frame/DOMWindow.cpp
+++ b/third_party/WebKit/Source/core/frame/DOMWindow.cpp
@@ -112,9 +112,10 @@ DOMWindow* DOMWindow::top() const {
   return frame()->tree().top()->domWindow();
 }
 
-External* DOMWindow::external() const {
-  DEFINE_STATIC_LOCAL(Persistent<External>, external, (new External));
-  return external;
+External* DOMWindow::external() {
+  if (!m_external)
+    m_external = new External;
+  return m_external;
 }
 
 DOMWindow* DOMWindow::anonymousIndexedGetter(uint32_t index) const {
@@ -446,6 +447,7 @@ void DOMWindow::focus(ExecutionContext* context) {
 DEFINE_TRACE(DOMWindow) {
   visitor->trace(m_frame);
   visitor->trace(m_location);
+  visitor->trace(m_external);
   EventTargetWithInlineData::trace(visitor);
 }