Add "own" symbols support.

src/factory.cc

Index: src/factory.cc
diff --git a/src/factory.cc b/src/factory.cc
index a52b95719db05276362f7325ec32be751b37d87a..832e34f1c76d374f0c6982b7bc4ec7cd7d7fe4fd 100644
--- a/src/factory.cc
+++ b/src/factory.cc
@@ -663,6 +663,14 @@ Handle<Symbol> Factory::NewPrivateSymbol() {
 }
 
 
+Handle<Symbol> Factory::NewPrivateOwnSymbol() {
+  Handle<Symbol> symbol = NewSymbol();
+  symbol->set_is_private(true);
+  symbol->set_is_own(true);
+  return symbol;
+}
+
+
 Handle<Context> Factory::NewNativeContext() {
   Handle<FixedArray> array = NewFixedArray(Context::NATIVE_CONTEXT_SLOTS);
   array->set_map_no_write_barrier(*native_context_map());