Add support for BeforeUnloadEvent

Source/bindings/v8/V8AbstractEventListener.cpp

Index: Source/bindings/v8/V8AbstractEventListener.cpp
diff --git a/Source/bindings/v8/V8AbstractEventListener.cpp b/Source/bindings/v8/V8AbstractEventListener.cpp
index f1f0c8aa7ea2b6925c1c6338417a4a8739945394..0164867d5592525511a5a553144580070e5259ce 100644
--- a/Source/bindings/v8/V8AbstractEventListener.cpp
+++ b/Source/bindings/v8/V8AbstractEventListener.cpp
@@ -37,6 +37,7 @@
 #include "bindings/v8/V8Binding.h"
 #include "bindings/v8/V8EventListenerList.h"
 #include "bindings/v8/V8HiddenPropertyName.h"
+#include "core/dom/BeforeUnloadEvent.h"
 #include "core/dom/Document.h"
 #include "core/dom/Event.h"
 #include "core/dom/EventNames.h"
@@ -157,8 +158,8 @@ void V8AbstractEventListener::invokeEventHandler(ScriptExecutionContext* context
     if (returnValue.IsEmpty())
         return;
 
-    if (!returnValue->IsNull() && !returnValue->IsUndefined() && event->storesResultAsString())
-        event->storeResult(toWebCoreString(returnValue));
+    if (event->type() == eventNames().beforeunloadEvent && !returnValue->IsNull() && !returnValue->IsUndefined())

[arv (Not doing code reviews), 2013/08/23 13:56:49]

This doesn't look safe. What if someone dispatches a non BeforeUnloadEvent with
that type (type is just a string).

var e = new Event('beforeunload', {bubbles: true});
document.body.dispatchEvent(e);

Maybe you can use interfaceName() instead?

[do-not-use, 2013/08/23 14:20:40]

Oops, you're right. I believe the most common pattern is to use a
isBeforeUnloadEvent() virtual method and use that.

[do-not-use, 2013/08/23 14:27:54]

Done.

+        static_cast<BeforeUnloadEvent*>(event)->setReturnValue(toWebCoreString(returnValue));
 
     if (m_isAttribute && shouldPreventDefault(returnValue))
         event->preventDefault();