Improve window.closed fidelity.

Source/core/frame/LocalDOMWindow.cpp

Index: Source/core/frame/LocalDOMWindow.cpp
diff --git a/Source/core/frame/LocalDOMWindow.cpp b/Source/core/frame/LocalDOMWindow.cpp
index 54881b1d527f87c67ca1461935e292a2decccbb5..5d40b65ab0da70d0e504d8517493cfba77f62851 100644
--- a/Source/core/frame/LocalDOMWindow.cpp
+++ b/Source/core/frame/LocalDOMWindow.cpp
@@ -817,6 +817,11 @@ void LocalDOMWindow::close(ExecutionContext* context)
     InspectorInstrumentation::willCloseWindow(context);
 
     page->chrome().closeWindowSoon();
+    // So as to make window.closed return the expected result
+    // after window.close(), separately record the to-be-closed
+    // state of this window. Scripts may access window.closed
+    // before the deferred close operation has gone ahead.
+    m_windowIsClosing = true;
 }
 
 void LocalDOMWindow::print()