GetCodeOwnersClient will return a GerritClient instance
if code-owners is enabled for the host, and will fall
back to DepotToolsClient otherwise.

Change-Id: I303147f36cb28ae3a40649fee020e6ea459b2c73
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2676989Reviewed-by: Gavin Mak <gavinmak@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>