Add support for BeforeUnloadEvent

Source/core/page/UseCounter.cpp

Index: Source/core/page/UseCounter.cpp
diff --git a/Source/core/page/UseCounter.cpp b/Source/core/page/UseCounter.cpp
index 885d9b88e0006ad823b63b46f061419362ed060a..4e16de2575e1f5027e55c0ea024d48eb77f71a60 100644
--- a/Source/core/page/UseCounter.cpp
+++ b/Source/core/page/UseCounter.cpp
@@ -666,6 +666,9 @@ String UseCounter::deprecationMessage(Feature feature)
     case FileError:
         return "FileError is deprecated. Please use the 'name' or 'message' attributes of DOMError rather than 'code'.";
 
+    case EventReturnValue:
+        return "event.returnValue is deprecated. Please use the standard event.preventDefault() instead.";
+
     // Features that aren't deprecated don't have a deprecation message.
     default:
         return String();