feat(Ads): Add ability to set tracking URL for Media Tailor

externs/shaka/ads.js

@@ -109,6 +109,11 @@ shaka.extern.IAdManager = class extends EventTarget {
    */
   requestMediaTailorStream(url, adsParams, backupUrl) {}
 
+  /**
+   * @param {string} url
+   */
+  addMediaTailorTrackingUrl(url) {}
+
   /**
    * @param {!HTMLElement} adContainer
    * @param {!HTMLMediaElement} video

lib/ads/ad_manager.js

@@ -560,6 +560,23 @@ shaka.ads.AdManager = class extends shaka.util.FakeEventTarget {
   }
 
 
+  /**
+   * @param {string} url
+   * @override
+   * @export
+   */
+  addMediaTailorTrackingUrl(url) {
+    if (!this.mtAdManager_) {
+      throw new shaka.util.Error(
+          shaka.util.Error.Severity.RECOVERABLE,
+          shaka.util.Error.Category.ADS,
+          shaka.util.Error.Code.MT_AD_MANAGER_NOT_INITIALIZED);
+    }
+
+    this.mtAdManager_.addTrackingUrl(url);
+  }
+
+
   /**
    * @override
    * @export

lib/ads/media_tailor_ad_manager.js

@@ -130,6 +130,15 @@ shaka.ads.MediaTailorAdManager = class {
     return this.streamPromise_;
   }
 
+  /**
+   * @param {string} url
+   */
+  addTrackingUrl(url) {
+    this.trackingUrl_ = url;
+    this.onEvent_(new shaka.util.FakeEvent(shaka.ads.AdManager.ADS_LOADED,
+        (new Map()).set('loadTime', 0)));
+  }
+
   /**
    * Resets the MediaTailor manager and removes any continuous polling.
    */

test/test/util/fake_ad_manager.js

@@ -47,6 +47,9 @@ shaka.test.FakeAdManager = class extends shaka.util.FakeEventTarget {
     return Promise.resolve('fake:url');
   }
 
+  /** @override */
+  addMediaTailorTrackingUrl(url) {}
+
   /** @override */
   initServerSide(adContainer, video) {}