DevTools: add use strict in check gn

Needed for node v4 (I previously tested node w/ v7 which doesn't need it).

BUG=none

Review-Url: https://codereview.chromium.org/2929163002
Cr-Commit-Position: refs/heads/master@{#478384}

third_party/WebKit/Source/devtools/scripts/check_gn.js

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
+'use strict';
 
 const fs = require('fs');
 const path = require('path');