startAbortableOperation_(action) {
    const promise = action.promise;
    this.openOperations_.push(promise);

    // Remove the open operation once the action has completed. So that we
    // can still return the AbortableOperation, this is done using a |finally|
    // block, rather than awaiting the result.
    return action.finally(() => {
      shaka.util.ArrayUtils.remove(this.openOperations_, promise);
    });
  }
The equivalent of startOperation_, but for abortable operations.

@param {!shaka.extern.IAbortableOperation<T>} action
@return {!shaka.extern.IAbortableOperation<T>}
@template T
@private