handleAppRequest(req, requestId) {
    // NOTE(koto): We pass an empty sender object. handleMessage_ does not
    // validate the sender anyway, as it's only listening for messages from the
    // current extension/app only.
    this.handleMessage_(
        req, /** @type {!MessageSender} */ ({}),
        goog.partial(this.sendResponse_, requestId));
  }
Handles requests sent from the Chrome App (Chrome App calls this function
directly).
@param {!e2e.ext.messages.GetSelectionRequest|
    !e2e.ext.messages.BridgeMessageResponse} req The request.
@param {string} requestId Identifier of the request to send it back in the
    response.
@export