resume() {
    this.pauseCount_--;
    if (!this.pauseCount_ && this.shouldFire_ && !this.timer_) {
      this.shouldFire_ = false;
      this.doAction_();
    }
  }
Resumes the throttle.  If doing so drops the pausing count to zero,
pending action callbacks will be executed as soon as possible, but
still no sooner than an interval's delay after the previous call.
Future action callbacks will be executed as normal.