cleanup: Abort all operations when destroying download manager

This was previously enforced only by the caller, but it makes more
sense to also enforce it at destroy(), which should always have the
last say in cancelation and cleanup.

Change-Id: I8c7a9f967383ed49104f673ba0ab239d28d69766

lib/offline/download_manager.js

@@ -44,9 +44,8 @@ shaka.offline.DownloadManager = class {
 
     /** @private {!shaka.util.Destroyer} */
     this.destroyer_ = new shaka.util.Destroyer(() => {
-      const promises = Array.from(this.groups_.values());
       // Add a "catch" block to stop errors from being returned.
-      return Promise.all(promises.map((p) => p.catch(() => {})));
+      return this.abortAll().catch(() => {});
     });
 
     /**