Add a flag to help platform ports bootstrap V8

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 8bfa12c519564dec459fba4c8a47b067eb35e2a9..761176d54a3ae8f83ca58b7f7301d07327fc9db2 100644
--- a/src/runtime/runtime-strings.cc
+++ b/src/runtime/runtime-strings.cc
@@ -272,9 +272,15 @@ RUNTIME_FUNCTION(Runtime_SubString) {
 RUNTIME_FUNCTION(Runtime_StringAdd) {
   HandleScope scope(isolate);
   DCHECK(args.length() == 2);
-  CONVERT_ARG_HANDLE_CHECKED(String, str1, 0);
-  CONVERT_ARG_HANDLE_CHECKED(String, str2, 1);
+  CONVERT_ARG_HANDLE_CHECKED(Object, obj1, 0);
+  CONVERT_ARG_HANDLE_CHECKED(Object, obj2, 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));
 }