[WebUI] Add ecmaVersion: 2017 to the root .eslintrc.js.

This CL adds a parser option to the root .eslintrc.js which is needed
for async/await. Since this is now allowed by the style guide, it makes
sense to enable the parser option everywhere.

Bug: 905927
Change-Id: I6a11032e38602a95c765709a3a998e5cd27c4875
Reviewed-on: https://chromium-review.googlesource.com/c/1343464
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: calamity <calamity@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613924}

.eslintrc.js

@@ -6,18 +6,23 @@ module.exports = {
   'root': true,
   'env': {
     'browser': true,
-    'es6': true
+    'es6': true,
+  },
+  'parserOptions': {
+    'ecmaVersion': 2017,
   },
   'rules': {
     // Enabled checks.
     'no-extra-semi': 'error',
     'no-new-wrappers': 'error',
-    'no-restricted-properties': ['error', {
-      'object': 'document',
-      'property': 'getElementById',
-      'message': 'Use $(\'id\') or getSVGElement(\'id\') ' +
-          'from chrome://resources/js/util.js instead of ' +
-          'document.getElementById(\'id\')',
+    'no-restricted-properties': [
+      'error',
+      {
+        'object': 'document',
+        'property': 'getElementById',
+        'message': 'Use $(\'id\') or getSVGElement(\'id\') ' +
+            'from chrome://resources/js/util.js instead of ' +
+            'document.getElementById(\'id\')',
       },
     ],
     'semi': ['error', 'always'],