selectAction_(contentBlob, evt) {
    this.removeChildren();

    if (!contentBlob) {
      contentBlob = /** @type {messages.BridgeMessageRequest} */ ({});
    }
    contentBlob.action =
        /** @type {constants.Actions} */ (evt.target.getValue());
    this.processSelectedContent_(contentBlob);
  }
Enables the user to select the PGP action they'd like to execute.
@param {?messages.BridgeMessageRequest} contentBlob The content that the
    user has selected.
@param {!goog.events.Event} evt The event generated by the user's
    selection.
@private