Add support for BeforeUnloadEvent

Source/core/page/UseCounter.h

 /*
  * Copyright (C) 2012 Google, Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 178 matching lines @@
         DocumentCharset, // Documented as IE extensions, from KHTML days.
         PrefixedAnimationEndEvent,
         UnprefixedAnimationEndEvent,
         PrefixedAndUnprefixedAnimationEndEvent,
         PrefixedAnimationStartEvent,
         UnprefixedAnimationStartEvent,
         PrefixedAndUnprefixedAnimationStartEvent,
         PrefixedAnimationIterationEvent,
         UnprefixedAnimationIterationEvent,
         PrefixedAndUnprefixedAnimationIterationEvent,
+        EventReturnValue, // Legacy IE extension.
         // Add new features immediately above this line. Don't change assigned
         // numbers of each items, and don't reuse unused slots.
         NumberOfFeatures, // This enum value must be last.
     };
 
     // "count" sets the bit for this feature to 1. Repeated calls are ignored.
     static void count(Document*, Feature);
     static void count(DOMWindow*, Feature);
     void count(CSSPropertyID);
     void count(Feature);
@@ skipping 35 matching lines @@
 
     void updateMeasurements();
 
     OwnPtr<BitVector> m_countBits;
     BitVector m_CSSFeatureBits;
 };
 
 } // namespace WebCore
 
 #endif // UseCounter_h