Make message listener API backwards compatible.

include/v8.h

Index: include/v8.h
diff --git a/include/v8.h b/include/v8.h
index bf3beff5f0d22a56efc7756845b34ffac130138b..c05d73b8f65569475af070bf2d996d094c38a0e7 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -3169,8 +3169,12 @@ class V8EXPORT V8 {
    *
    * The same message listener can be added more than once and in that
    * case it will be called more than once for each message.
+   *
+   * If data is specified, it will be passed to the callback when it is called.
+   * Otherwise, the exception object will be passed to the callback instead.
    */
-  static bool AddMessageListener(MessageCallback that);
+  static bool AddMessageListener(MessageCallback that,
+                                 Handle<Value> data = Handle<Value>());
 
   /**
    * Remove all message listeners from the specified callback function.