Add a TurboFan skeleton for StringAddStub.

src/runtime.js

Index: src/runtime.js
diff --git a/src/runtime.js b/src/runtime.js
index d89715a1b5915994eb25241a02f9c5f22b829a73..86db49595efc179359048f3f74b45d6925aa478a 100644
--- a/src/runtime.js
+++ b/src/runtime.js
@@ -65,6 +65,7 @@ var TO_STRING;
 var TO_NAME;
 
 var StringLengthTF_STUB;
+var StringAddTF_STUB;
 var MathFloor_STUB;
 
 var $defaultNumber;
@@ -772,6 +773,10 @@ StringLengthTF_STUB = function StringLengthTF_STUB(receiver, name, i, v) {
   return %_StringGetLength(%_JSValueGetValue(receiver));
 }
 
+StringAddTF_STUB = function StringAddTF_STUB(left, right) {
+  return %StringAdd(left, right);
+}
+
 MathFloor_STUB = function MathFloor_STUB(f, i, v) {
   // |f| is calling function's JSFunction
   // |i| is TypeFeedbackVector slot # of callee's CallIC for Math.floor call