DevTools: add use strict in check gn

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');
 
 const inspectorManifest = require('../front_end/inspector.json');
 const utils = require('./utils');
 
 const gnPath = path.resolve(__dirname, '..', 'BUILD.gn');
 const gnFile = fs.readFileSync(gnPath, 'utf-8');
 const gnLines = gnFile.split('\n');
@@ skipping 42 matching lines @@
 function selectGNLines(startLine, endLine) {
   let lines = gnLines.map(line => line.trim());
   let startIndex = lines.indexOf(startLine);
   if (startIndex === -1)
     return [];
   let endIndex = lines.indexOf(endLine, startIndex);
   if (endIndex === -1)
     return [];
   return lines.slice(startIndex + 1, endIndex);
 }