[fenix] For https://github.com/mozilla-mobile/fenix/issues/6558 - change from modifying event to event listener

The initial design replaced the onload method, which was used by others such as Bing.
With listeners it's additive, we do not replace anything.

app/src/main/assets/extensions/ads/ads.js

@@ -2,7 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
- const ADLINK_CHECK_TIMEOUT_MS = 1000;
+const ADLINK_CHECK_TIMEOUT_MS = 1000;
 
 function collectLinks(urls) {
     let anchors = document.getElementsByTagName("a");
@@ -25,18 +25,26 @@ function sendLinks() {
     browser.runtime.sendNativeMessage("MozacBrowserAds", message);
 }
 
+const events = ["pageshow", "load", "unload"];
 var timeout;
 
-window.onload = function() {
-    timeout = setTimeout(sendLinks, ADLINK_CHECK_TIMEOUT_MS);
-};
-
-window.onpageshow = function(event) {
-    if (event.persisted) {
+const eventLogger = event => {
+  switch (event.type) {
+    case "load":
         timeout = setTimeout(sendLinks, ADLINK_CHECK_TIMEOUT_MS);
-    }
+        break;
+    case "pageshow":
+        if (event.persisted) {
+          timeout = setTimeout(sendLinks, ADLINK_CHECK_TIMEOUT_MS);
+        }
+        break;
+    case "unload":
+        clearTimeout(timeout);
+    default:
+        console.log('Event:', event.type);
+  }
 };
 
-window.onunload = function() {
-  clearTimeout(timeout);
-};
+events.forEach(eventName =>
+  window.addEventListener(eventName, eventLogger)
+);