R1:8cbfb386bb87

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