When evaluating the "Branches" Herald field from the test console, use the current viewer

Summary: Ref T13093. Depends on D19145. See PHI398. Previously, see D18933. This provides the current viewer to ConduitCall so that we don't try to use device credentials from unprivileged web hosts.

Test Plan: Evaluated the "Branches" field locally, saw an appropriate field value.

Maniphest Tasks: T13093

Differential Revision: https://secure.phabricator.com/D19146