Cache result of ListOwners to avoid making duplicate REST API calls.

Change-Id: Ie97fd7b605b73e1baed74302615f9df44fb622d2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2693914
Auto-Submit: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>