Make USE_KERBEROS use the BUILDFLAG system, and make services/ use it.

http_auth_handler_negotiate.cc is only included if use_kerberos is set,
so its tests should only be included then too.

Since the existing USE_KERBEROS define was internal to net, use the
BUILDFLAG system to make it accessible outside of net.

use_kerberos is true on Android but the test used to not run on Android.
Maybe used should be false on Android, but for now this preserves behavior.

This makes services_unittests link on the Cast bots, enabling
https://chromium-review.googlesource.com/c/chromium/src/+/1128939

No behavior change.

Bug: 843511
Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: Ic363bcd7cac928bc323f3872d7beaa827fc9a1cc
Reviewed-on: https://chromium-review.googlesource.com/1129259
Reviewed-by: Helen Li <xunjieli@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573336}
5 files changed