onWebkitKeyAdded_(event) {
    shaka.log.debug('PatchedMediaKeysApple.onWebkitKeyAdded_', event);

    // This shouldn't fire while we're in the middle of generateRequest,
    // but if it does, we will need to change the logic to account for it.
    goog.asserts.assert(!this.generateRequestPromise_,
        'Key added during generate!');

    // We can now resolve this.updatePromise, which should be non-null.
    goog.asserts.assert(this.updatePromise_,
        'updatePromise_ should be set before now!');
    if (this.updatePromise_) {
      this.updateKeyStatus_('usable');
      this.updatePromise_.resolve();
      this.updatePromise_ = null;
    }
  }
Handler for the native keyadded event on WebKitMediaKeySession.

@param {!MediaKeyEvent} event
@private