isBuffering() {
    const State = shaka.media.BufferingObserver.State;
    return this.bufferObserver_ ?
           this.bufferObserver_.getState() == State.STARVING :
           !!this.assetUri_;
  }
Check if the player is currently in a buffering state (has too little
content to play smoothly). If the player has not loaded content, this will
return <code>false</code>.

@return {boolean}
@export