startGlassListener_() {
    if (this.isGmail_() && !window.ENABLED_LOOKING_GLASS) {
      this.activeViewListenerKey_ = goog.events.listen(
          document.body, goog.events.EventType.CLICK, this.enableLookingGlass_,
          true, this);
    }

    /** @type {boolean} */
    window.ENABLED_LOOKING_GLASS = true;
  }
Attaches click listener enabling a looking glass for current message in
    Gmail.
@private