[cleanup] The %StringAdd runtime function is always called on Strings.

src/runtime/runtime-strings.cc

Index: src/runtime/runtime-strings.cc
diff --git a/src/runtime/runtime-strings.cc b/src/runtime/runtime-strings.cc
index 3a435913e35b15f2f8dfbd4eafe9800737f62f50..e2c05c1d5ab6853bf7c8ea860ad2fc3eb3044f17 100644
--- a/src/runtime/runtime-strings.cc
+++ b/src/runtime/runtime-strings.cc
@@ -191,15 +191,9 @@ RUNTIME_FUNCTION(Runtime_SubString) {
 RUNTIME_FUNCTION(Runtime_StringAdd) {
   HandleScope scope(isolate);
   DCHECK_EQ(2, args.length());
-  CONVERT_ARG_HANDLE_CHECKED(Object, obj1, 0);
-  CONVERT_ARG_HANDLE_CHECKED(Object, obj2, 1);
+  CONVERT_ARG_HANDLE_CHECKED(String, str1, 0);
+  CONVERT_ARG_HANDLE_CHECKED(String, str2, 1);
   isolate->counters()->string_add_runtime()->Increment();
-  MaybeHandle<String> maybe_str1(Object::ToString(isolate, obj1));
-  MaybeHandle<String> maybe_str2(Object::ToString(isolate, obj2));
-  Handle<String> str1;
-  Handle<String> str2;
-  maybe_str1.ToHandle(&str1);
-  maybe_str2.ToHandle(&str2);
   RETURN_RESULT_OR_FAILURE(isolate,
                            isolate->factory()->NewConsString(str1, str2));
 }