DevTools: check BUILD.gn file as part of PRESUBMIT

third_party/WebKit/Source/devtools/package.json

 {
   "name": "chrome-devtools-frontend",
   "description": "Chrome DevTools UI",
   "scripts": {
     "start": "node scripts/start_chrome_and_server.js",
     "chrome": "node scripts/chrome_debug_launcher/launch_chrome.js",
     "server": "node scripts/hosted_mode/server.js",
     "test": "node scripts/npm_test.js",
     "debug-test": "node scripts/npm_test.js --debug-devtools",
     "compat-test": "node scripts/npm_test.js --compat-protocol=1.2",
     "lint": "eslint front_end",
     "format": "git cl format --js .",
     "closure": "python scripts/compile_frontend.py",
     "setup-dtrun": "cd scripts/devtools_run && npm link",
     "format-py": "yapf --exclude scripts/build/rjsmin.py -i --recursive scripts PRESUBMIT.py",
-    "extract": "node scripts/extract_module/extract_module.js"
+    "extract": "node scripts/extract_module/extract_module.js",
+    "check-gn": "node scripts/check_gn.js"
   },
   "repository": {
     "type": "git",
     "url": "git+https://github.com/ChromeDevTools/devtools-frontend.git"
   },
   "keywords": [
     "devtools",
     "chrome",
     "chromium",
     "blink",
     "debugger"
   ],
   "author": "The Chromium Authors",
   "license": "SEE LICENSE IN https://chromium.googlesource.com/chromium/src/+/master/LICENSE",
   "bugs": {
     "url": "https://bugs.chromium.org/p/chromium/issues/list?can=2&q=component:Platform%3EDevTools%20&sort=-opened&colspec=ID%20Stars%20Owner%20Summary%20Modified%20Opened"
   },
   "homepage": "https://devtools.chrome.com",
   "devDependencies": {
     "eslint": "3.10.0"
   }
 }