[turbofan] Implement JSBuiltinReducer for String.prototype[Symbol.iterator].

src/bootstrapper.cc

Index: src/bootstrapper.cc
diff --git a/src/bootstrapper.cc b/src/bootstrapper.cc
index 6f799e2599aef030cf449847593b53787c8a8f02..192d3700885930a2780214c80c46f67ca59be10d 100644
--- a/src/bootstrapper.cc
+++ b/src/bootstrapper.cc
@@ -1476,6 +1476,7 @@ void Genesis::InitializeGlobal(Handle<JSGlobalObject> global_object,
         isolate, factory->NewStringFromAsciiChecked("[Symbol.iterator]"),
         Builtins::kStringPrototypeIterator, 0, true);
     iterator->shared()->set_native(true);
+    iterator->shared()->set_builtin_function_id(kStringIterator);
     JSObject::AddProperty(prototype, factory->iterator_symbol(), iterator,
                           static_cast<PropertyAttributes>(DONT_ENUM));
   }