Only gerrit_url is needed to initialize gerrit_obj.
This will be needed for git cl to call presubmit support
as a script.

Bug: 1042324
Change-Id: I79d78fb67465f989755bf8c8ceb48d665464fa05
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2005050Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>