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

There's no point in trying to convert to a string, as this runtime
function must always be called with Strings, otherwise there's a bug.

R=yangguo@chromium.org

Review-Url: https://codereview.chromium.org/2748253004
Cr-Commit-Position: refs/heads/master@{#43830}

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));
 }