handleExternalMessage_(event) {
    if (!this.isPopout || (event.origin !== location.origin) ||
        this.wasDecorated()) {
      return;
    }
    var request =
        /**@type {?e2e.ext.messages.BridgeMessageRequest} */ (event.data);
    this.injectContent(request);
    this.decorate(document.body);
    window.removeEventListener('message', this.boundMessageListener_);
  }
Handles external messages to initialize the popout prompt. Will unbind
itself once the message is processed.
@param  {!Event} event Incoming event.
@private