presubmit_support: Initialize Gerrit object when gerrit URL is given.
presubmit_support: Initialize Gerrit object when gerrit URL is given.

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/+/2005050
Reviewed-by: Anthony Polito <05bb588aa9020353543fee7ac4e6e0a3d0d6f6ed@google.com>
Commit-Queue: Edward Lesmes <d05609f027dc667cc37492b1dc1bdea5d60fa82a@chromium.org>

CoherentLabs/depot_tools,CoherentLabs/depot_tools