R1:29bdc3ffc5a4

Fix DifferentialRevisionEditor handling of actorPHID after D3645

Summary: `actorPHID` no longer gets set or exists.

Test Plan: Updated a revision without fataling.

Reviewers: btrahan, vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D3684