toggleKeyListenener_(event) {
    if (this.isPromptDisplayed_()) {
      return;  // Disallow toggling when the prompt is displayed.
    }
    // Detect toggle key.
    if (event instanceof KeyboardEvent) {
      if (event.keyCode == goog.events.KeyCodes.CTRL &&
          (event.keyIdentifier == 'U+00A2' ||    // OSX
           event.keyIdentifier == 'Control')) {  // Left Ctrl
        if (event.type == goog.events.EventType.KEYDOWN) {
          this.toggleInteractionSuppressKeyDown_ = true;
        } else if (
            event.type == goog.events.EventType.KEYUP &&
            this.toggleInteractionSuppressKeyDown_) {
          this.toggleInteractionSuppressKeyDown_ = false;
          this.toggleInteractionSuppressedMode_();
          return;
        }
      } else {
        this.toggleInteractionSuppressKeyDown_ = false;  // Some other key.
      }
    }
  }
Toggles the interaction suppress mode if the toggle key is tapped.
@param  {!Event} event
@private