onMouseStill_() {
    // Hide the cursor.
    this.videoContainer_.classList.add('no-cursor');
    this.recentMouseMovement_ = false;
    this.computeOpacity();
  }
This callback is for when we are pretty sure that the mouse has stopped
moving (aka the mouse is still). This method should only be called via
|mouseStillTimer_|. If this behaviour needs to be invoked directly, use
|mouseStillTimer_.tickNow()|.

@private