Further improve google-ima shim script

I worked through some of the websites listed in the uBlock issue[1]
for the google-ima.js surrogate script, to see what was going
wrong. It turned out the addEventListener method supports an optional
context Object, which is bound to the listener if provided. Some
websites make use of that, and then break when `this` is not bound
correctly when events are dispatched.

See also duckduckgo/tracker-surrogates#24

1 - uBlockOrigin/uBlock-issues#2265

src/web_accessible_resources/google-ima.js

@@ -9,6 +9,10 @@
  * - Modified to avoid jshint warnings as per uBO's config
  * - Added `OmidVerificationVendor` to `ima`
  * - Have `AdError.getInnerError()` return `null`
+ * - Have `AdDisplayContainer` constructor add DIV element to container
+ * - Added missing event dispatcher functionality
+ * - Corrected return type of `Ad.getUniversalAdIds()`
+ * - Corrected typo in `UniversalAdIdInfo.getAdIdValue()` method name
  * 
  * Related issue:
  * - https://github.com/uBlockOrigin/uBlock-issues/issues/2158
@@ -331,8 +335,9 @@ if (!window.google || !window.google.ima || !window.google.ima.VERSION) {
     }
 
     _dispatch(e) {
-      const listeners = this.listeners.get(e.type) || [];
-      for (const listener of Array.from(listeners)) {
+      let listeners = this.listeners.get(e.type);
+      listeners = listeners ? Array.from(listeners.values()) : [];
+      for (const listener of listeners) {
         try {
           listener(e);
         } catch (r) {
@@ -341,16 +346,16 @@ if (!window.google || !window.google.ima || !window.google.ima.VERSION) {
       }
     }
 
-    addEventListener(types, c) {
+    addEventListener(types, c, options, context) {
       if (!Array.isArray(types)) {
         types = [types];
       }
 
       for (const t of types) {
         if (!this.listeners.has(t)) {
-          this.listeners.set(t, new Set());
+          this.listeners.set(t, new Map());
         }
-        this.listeners.get(t).add(c);
+        this.listeners.get(t).set(c, c.bind(context || this));
       }
     }