ready() {
    shaka.log.debug('PatchedMediaKeysWebkit.MediaKeySession.ready');

    this.updateKeyStatus_('usable');

    if (this.updatePromise_) {
      this.updatePromise_.resolve();
    }
    this.updatePromise_ = null;
  }
Signals that the session is 'ready', which is the terminology used in older
versions of EME.  The new signal is to resolve the 'update' promise.  This
translates between the two.

@protected