Fix scoping issue when running the DOM tests

content_scripts/vimium_frontend.js

@@ -2,7 +2,7 @@
 // This content script must be run prior to domReady so that we perform some operations very early.
 //
 
-let root = typeof exports !== 'undefined' && exports !== null ? exports : (window.root != null ? window.root : (window.root = {}));
+const root = {};
 // On Firefox, sometimes the variables assigned to window are lost (bug 1408996), so we reinstall them.
 // NOTE(mrmr1993): This bug leads to catastrophic failure (ie. nothing works and errors abound).
 DomUtils.documentReady(function() {
@@ -207,6 +207,7 @@ const initializePreDomReady = function() {
 
 // Wrapper to install event listeners.  Syntactic sugar.
 const installListener = (element, event, callback) => element.addEventListener(event, forTrusted(function() {
+  // TODO(philc): I think this workaround can be removed?
   if (typeof global === 'undefined' || global === null) { // See #2800.
     Object.assign(window, root);
   }

tests/dom_tests/chrome.js

@@ -2,8 +2,7 @@
 // Mock the Chrome extension API.
 //
 
-let root = window;
-root.chromeMessages = [];
+window.chromeMessages = [];
 
 document.hasFocus = () => true;
 
@@ -13,7 +12,7 @@ const fakeManifest = {
   version: "1.51"
 };
 
-root.chrome = {
+window.chrome = {
   runtime: {
     connect() {
       return {