Upstream the changes from: Bug 1339461 - Convert foo.indexOf(...) == -1 to foo.includes() and implement an eslint rule to enforce this

Yet another case where PDF.js code was modified in `mozilla-central` without the changes happening in the GitHub repo first; *sigh*.
If we don't upstream at least the changes in `extensions/firefox/`, any future update of PDF.js in `mozilla-central` will be blocked.

Please see:
 - https://bugzilla.mozilla.org/show_bug.cgi?id=1339461
 - https://hg.mozilla.org/mozilla-central/rev/d5a5ad1dbbf2

src/display/dom_utils.js

@@ -229,7 +229,7 @@ class SimpleXMLParser {
   }
 
   _decodeXML(text) {
-    if (text.indexOf('&') < 0) {
+    if (!text.includes('&')) {
       return text;
     }