fixup drm engine tests

lib/media/drm_engine.js

@@ -389,8 +389,9 @@ shaka.media.DrmEngine = class {
   /**
    * Attach MediaKeys to the video element
    * @return {!Promise}
+   * @private
    */
-  async attachMediaKeys() {
+  async attachMediaKeys_() {
     if (this.video_.mediaKeys) {
       return;
     }
@@ -406,21 +407,22 @@ shaka.media.DrmEngine = class {
           shaka.util.Error.Code.FAILED_TO_ATTACH_TO_VIDEO,
           exception.message));
     }
+
+    this.destroyer_.ensureNotDestroyed();
   }
 
   /**
    * Processes encrypted event and start licence challenging
    * @return {!Promise}
+   * @private
    */
-  async onEncryptedEvent(event) {
+  async onEncryptedEvent_(event) {
     /**
      * MediaKeys should be added when receiving an encrypted event. Setting
      * mediaKeys before could result into encrypted event not being fired on
      * some browsers
      */
-    await this.attachMediaKeys();
-
-    this.destroyer_.ensureNotDestroyed();
+    await this.attachMediaKeys_();
 
     this.newInitData(
         event.initDataType,
@@ -461,25 +463,29 @@ shaka.media.DrmEngine = class {
           () => this.closeOpenSessions_());
     }
 
+    const nonEmptyInitData = this.currentDrmInfo_.initData.find(
+        (initDataOverride) => initDataOverride.initData.length > 0);
+
     /**
      * Legacy implementations requires MediaKeys to be set before having
      * webkitneedkey / msneedkey event, which will be translated as an
      * encrypted event by the polyfills
+     *
+     * If some initData already has been generated, we need to attach media
+     * keys so we can start session before playback actually begins
      */
-    if (shaka.util.Platform.isMediaKeysPolyfilled()) {
-      await this.attachMediaKeys();
+    if (nonEmptyInitData || shaka.util.Platform.isMediaKeysPolyfilled()) {
+      await this.attachMediaKeys_();
     }
 
-    this.destroyer_.ensureNotDestroyed();
-
     this.createOrLoad();
 
     // Explicit init data an offline session is
     // sufficient to suppress 'encrypted' events for all streams.
     if (!this.offlineSessionIds_.length) {
       this.eventManager_.listen(
           // eslint-disable-next-line no-restricted-syntax
-          this.video_, 'encrypted', this.onEncryptedEvent.bind(this));
+          this.video_, 'encrypted', this.onEncryptedEvent_.bind(this));
     }
   }