Fix waterfall bots to not use fastbuild and add presubmit validation.

Fix waterfall bots to not use fastbuild and add presubmit validation.

Previously both tryservers and waterfall GYP bots were being built w/
fastbuild=1, but we want the non-windows waterfall bots to have
as much debug info as possible. Windows bots should still be
built w/ fastbuild=1, because they take too long to link otherwise.

Also, fix an oversight where the debug waterfall
bots were not configured to use swarming

Also, we would validate that the mb_config.pyl config file was
correct in a unit test, but that wasn't very obvious and now we
have presubmit checks for MB, so move this to a presubmit check
instead.

R=jam@chromium.org
BUG=481692
CQ_EXTRA_TRYBOTS=tryserver.chromium.mac:mac_chromium_gn_rel,mac_chromium_gn_dbg

Review URL: https://codereview.chromium.org/1281773004

Cr-Commit-Position: refs/heads/master@{#343024}