Add support for BeforeUnloadEvent

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 9eae59b33bc68d1871fca5c874997b604cf72652..32efe9ac61556c692b16f04bd2e9fdd09a945a44 100644
--- a/Source/core/dom/Document.cpp
+++ b/Source/core/dom/Document.cpp
@@ -2337,7 +2337,7 @@ bool Document::dispatchBeforeUnloadEvent(Chrome& chrome, Document* navigatingDoc
     m_loadEventProgress = BeforeUnloadEventCompleted;
     if (!beforeUnloadEvent->defaultPrevented())
         defaultEventHandler(beforeUnloadEvent.get());
-    if (beforeUnloadEvent->result().isNull())
+    if (beforeUnloadEvent->returnValue().isNull())
         return true;
 
     if (navigatingDocument->m_didAllowNavigationViaBeforeUnloadConfirmationPanel) {
@@ -2345,7 +2345,7 @@ bool Document::dispatchBeforeUnloadEvent(Chrome& chrome, Document* navigatingDoc
         return true;
     }
 
-    String text = displayStringModifiedByEncoding(beforeUnloadEvent->result());
+    String text = displayStringModifiedByEncoding(beforeUnloadEvent->returnValue());
     if (chrome.runBeforeUnloadConfirmPanel(text, m_frame)) {
         navigatingDocument->m_didAllowNavigationViaBeforeUnloadConfirmationPanel = true;
         return true;