setContentImage(imageUrl) {
    if (!this.metadata_) {
      this.metadata_ = {
        'metadataType': cast.receiver.media.MetadataType.GENERIC,
      };
    }
    this.metadata_['images'] = [
      {
        'url': imageUrl,
      },
    ];
  }
Set the Cast content's thumbnail image.
Should be called from an appDataCallback.

@param {string} imageUrl
@export