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

src/bootstrapper.cc

Index: src/bootstrapper.cc
diff --git a/src/bootstrapper.cc b/src/bootstrapper.cc
index 16e77300a03ab6ba622b11a0d403dee8fa7bb42e..9ba985a759520326b181788f2e7b586b327c3b44 100644
--- a/src/bootstrapper.cc
+++ b/src/bootstrapper.cc
@@ -1442,6 +1442,7 @@ 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(kStringIteratorPrototypeNext);
 
     // Set the expected parameters for %StringIteratorPrototype%.next to 0 (not
     // including the receiver), as required by the builtin.