Prevent firing BeforeUnloadEvents re-entrantly

Source/core/dom/Document.cpp

Index: Source/core/dom/Document.cpp
diff --git a/Source/core/dom/Document.cpp b/Source/core/dom/Document.cpp
index 5dff7d9be6b17f60c61c6dafcbe06da45d9792d9..c9df01d4efa2414631c9617e538ca2be19576d82 100644
--- a/Source/core/dom/Document.cpp
+++ b/Source/core/dom/Document.cpp
@@ -2581,6 +2581,9 @@ bool Document::dispatchBeforeUnloadEvent(Chrome& chrome, bool& didAllowNavigatio
     if (!body())
         return true;
 
+    if (processingBeforeUnload())
+        return false;
+
     RefPtrWillBeRawPtr<Document> protect(this);
 
     RefPtrWillBeRawPtr<BeforeUnloadEvent> beforeUnloadEvent = BeforeUnloadEvent::create();