handleMessage_(req, sender, sendResponse) {
    if (goog.object.containsKey(req, 'enableLookingGlass')) {
      this.getSelectedContent_(/** @type {?} */ (req), sendResponse);
    } else if (goog.object.containsKey(req, 'value')) {
      this.setValue_(/** @type {?} */ (req), sendResponse);
    }
    return true;
  }
Handles messages sent from the extension.
@param {!e2e.ext.messages.GetSelectionRequest|
    !e2e.ext.messages.BridgeMessageResponse} req The request.
@param {!MessageSender} sender The sender of the request.
@param {function(*)} sendResponse A callback function sending the response
    back.
@return {boolean} Returns true to mark that the response is sent
    asynchronously.
@private