reset() {
    return GoogPromise.resolve().then(() => {
      if (this.ui_) {
        this.ui_.delete();
      }
    }).then(() => {
      this.ui_ = null;
      this.disposeCurrentComponent_();
    });
  }
Resets the FirebaseUI handler and deletes the underlying FirebaseUI
instance. Calling startSignIn after reset should rerender the UI
successfully.
@return {!GoogPromise<void>} The promise that resolves when the instance is
    successfully reset.