Bug 1875244 - Enable ESlint rules no-unused-vars and no-undef for dom/xslt. r=peterv

Differential Revision: https://phabricator.services.mozilla.com/D198936

.eslintrc.js

@@ -514,7 +514,6 @@ module.exports = {
         "dom/websocket/tests/**",
         "dom/workers/test/**",
         "dom/worklet/tests/**",
-        "dom/xslt/tests/**",
       ],
       rules: {
         "no-undef": "off",

dom/xslt/tests/mochitest/file_bug1729517.js

@@ -1,3 +1,4 @@
+/* global fail */
 fail(
   "documents sandboxed without allow-scripts should NOT be able to run <script src=...>"
 );

dom/xslt/tests/mochitest/test_bug1729517.html

@@ -14,7 +14,7 @@
           let failed = JSON.parse(frame.contentDocument.title);
           ok(failed instanceof Array, "Frame's title is expected to be a JSON representation of the array of failed conditions.");
           is(failed.length, 0, "No scripts should run in sandboxed iframe document created by XSLT.");
-          for (desc of failed) {
+          for (let desc of failed) {
             info(desc);
           }
           resolve();