Rename a `setViewer` call to `setUser` to fix a fatal.

Summary: Fixes T2303.

Test Plan: Rendered /uiexample/view/PhabricatorTimelineExample/ without a fatal

Reviewers: chad, epriestley, btrahan

Reviewed By: btrahan

CC: aran, Korvin

Maniphest Tasks: T2303

Differential Revision: https://secure.phabricator.com/D4387