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

src/code-factory.cc

Index: src/code-factory.cc
diff --git a/src/code-factory.cc b/src/code-factory.cc
index 6835ba0eda909a6ca4600502c598184002d4ff77..c3da10532301274e359c40bf8ca49cee7446e2a4 100644
--- a/src/code-factory.cc
+++ b/src/code-factory.cc
@@ -176,6 +176,13 @@ Callable CodeFactory::ToNumber(Isolate* isolate) {
 
 
 // static
+Callable CodeFactory::ToString(Isolate* isolate) {
+  ToStringStub stub(isolate);
+  return Callable(stub.GetCode(), stub.GetCallInterfaceDescriptor());
+}
+
+
+// static
 Callable CodeFactory::ToObject(Isolate* isolate) {
   ToObjectStub stub(isolate);
   return Callable(stub.GetCode(), stub.GetCallInterfaceDescriptor());