[runtime] Add %ToString and %_ToString and remove the TO_STRING builtin.

src/runtime/runtime-object.cc

Index: src/runtime/runtime-object.cc
diff --git a/src/runtime/runtime-object.cc b/src/runtime/runtime-object.cc
index cb5b3b685a6e1168ef92770def9b6dd3fd9262c7..aa2e9c0744bd696a9fbbe24685f7d865404a8d47 100644
--- a/src/runtime/runtime-object.cc
+++ b/src/runtime/runtime-object.cc
@@ -1484,6 +1484,17 @@ RUNTIME_FUNCTION(Runtime_ToNumber) {
 }
 
 
+RUNTIME_FUNCTION(Runtime_ToString) {
+  HandleScope scope(isolate);
+  DCHECK_EQ(1, args.length());
+  CONVERT_ARG_HANDLE_CHECKED(Object, input, 0);
+  Handle<Object> result;
+  ASSIGN_RETURN_FAILURE_ON_EXCEPTION(isolate, result,
+                                     Object::ToString(isolate, input));
+  return *result;
+}
+
+
 RUNTIME_FUNCTION(Runtime_ToName) {
   HandleScope scope(isolate);
   DCHECK_EQ(1, args.length());