WebUI: Hook up ESLint to web_dev_style PRESUBMIT.

.eslintrc.js

Index: .eslintrc.js
diff --git a/.eslintrc.js b/.eslintrc.js
new file mode 100644
index 0000000000000000000000000000000000000000..9b4b1037ce4fc136daafe0e66419f9a5ce13f2cb
--- /dev/null
+++ b/.eslintrc.js
@@ -0,0 +1,16 @@
+// 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.
+
+module.exports = {
+  'root': true,
+  'env': {
+    'browser': true,
+    'es6': true
+  },
+  'rules': {
+    // Enabled checks.
+    'no-extra-semi': 'error',
+    // TODO(dpapad): Add more checks according to our styleguide.
+  },
+};