hasValidPosition() {
    if (this.type == 'FIELD' && this.blockId && this.fieldName) {
      return true;
    } else if (this.type == 'BLOCK' && this.blockId) {
      return true;
    } else {
      return false;
    }
  }
Check if the combination of Position properties describe a viable position.
@returns {!Boolean} Whether the PositionUpdate has a viable position.
@public