Add NativeWeakMap to v8.h

src/factory.cc

Index: src/factory.cc
diff --git a/src/factory.cc b/src/factory.cc
index ce99df2e08cf243716496b06263b07d5d640af04..0010845a3937e3c34992859db347ee83eff4efa2 100644
--- a/src/factory.cc
+++ b/src/factory.cc
@@ -2237,6 +2237,15 @@ Handle<JSObject> Factory::NewArgumentsObject(Handle<JSFunction> callee,
 }
 
 
+Handle<JSWeakMap> Factory::NewJSWeakMap() {
+  // TODO(adamk): Currently the map is only created three times per
+  // isolate. If it's created more often, the map should be moved into the
+  // strong root list.
+  Handle<Map> map = NewMap(JS_WEAK_MAP_TYPE, JSWeakMap::kSize);
+  return Handle<JSWeakMap>::cast(NewJSObjectFromMap(map));
+}
+
+
 Handle<Map> Factory::ObjectLiteralMapFromCache(Handle<Context> context,
                                                int number_of_properties,
                                                bool* is_result_from_cache) {