This change implements ListOwnersForFile using gerrit's REST API in owners_client.py
Change-Id: Ic454dcd89a8f00f6b9e913e6b8ee9e1169581b38
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2583327Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Gavin Mak <gavinmak@google.com>