[turbofan] inline %StringIteratorPrototype%.next in JSBuiltinReducer.

src/bootstrapper.cc

Index: src/bootstrapper.cc
diff --git a/src/bootstrapper.cc b/src/bootstrapper.cc
index 0189e77d15427161350a4df5bc7faf036ad52d17..e2a90deb5f65ec3961498061f0d1106c388dcf40 100644
--- a/src/bootstrapper.cc
+++ b/src/bootstrapper.cc
@@ -1450,6 +1450,7 @@ void Genesis::InitializeGlobal(Handle<JSGlobalObject> global_object,
         InstallFunction(string_iterator_prototype, "next", JS_OBJECT_TYPE,
                         JSObject::kHeaderSize, MaybeHandle<JSObject>(),
                         Builtins::kStringIteratorPrototypeNext);
+    next->shared()->set_builtin_function_id(kStringIteratorNext);
 
     // Set the expected parameters for %StringIteratorPrototype%.next to 0 (not
     // including the receiver), as required by the builtin.