Fix the "PersistentValueMap" memory leak reported here:

include/v8-util.h

Index: include/v8-util.h
diff --git a/include/v8-util.h b/include/v8-util.h
index c58f4cab5c16018839f95bfb32d9879639d8b875..86549252b836bbad8c89d8aa667245c683b4bbff 100644
--- a/include/v8-util.h
+++ b/include/v8-util.h
@@ -300,6 +300,7 @@ class PersistentValueMap {
       K key = Traits::KeyFromWeakCallbackData(data);
       Traits::Dispose(data.GetIsolate(),
                       persistentValueMap->Remove(key).Pass(), key);
+      Traits::DisposeCallbackData(data.GetParameter());
     }
   }
 
@@ -337,7 +338,7 @@ class PersistentValueMap {
   static UniquePersistent<V> Release(PersistentContainerValue v) {
     UniquePersistent<V> p;
     p.val_ = FromVal(v);
-    if (Traits::kCallbackType != kNotWeak && !p.IsEmpty()) {
+    if (Traits::kCallbackType != kNotWeak && p.IsWeak()) {
       Traits::DisposeCallbackData(
           p.template ClearWeak<typename Traits::WeakCallbackDataType>());
     }