This CL basically replaces "skip if is_gce" with "skip if default auth method
is not gitcookies".

This presumable makes 'git cl' work on LUCI in a consistent manner. Before, it
worked only if the LUCI bot happened to also be GCE bot.

R=tandrii@chromium.org
BUG=891755

Change-Id: I2caa219a4082438a5e026e728bfb62f46a0c80fd
Reviewed-on: https://chromium-review.googlesource.com/c/1260053
Commit-Queue: Vadim Shtayura <vadimsh@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>