Made some additional methods of ArcanistBaseWorkflow final.

Summary: To me, it seems that these methods should never be overwritten in subclasses.

Test Plan: arc lint and arc unit.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: LegNeato, aran, epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D7959

Conflicts:
src/workflow/ArcanistBaseWorkflow.php