my_activity can definitely be useful outside of Chromium, and the
gerrit instances are sometimes private. This allows users to carry
an additional list of instances in a json file (and possibly
monorail projects, but that seems like likely to be useful).

Change-Id: Ic0fec3bd4f815200f27cc0a8530cfd88f4328269
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2925677
Commit-Queue: Nicolas Boichat <drinkcat@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>