Convert sandbox_mac_unittests to run exclusively on Swarming

Also add gn_unittests_run to build/gn_migration.gypi, which
I forgot in https://codereview.chromium.org/1214533005/

Also reshuffle build/gn_migration.gypi to use consistent conditionals.

0 configs already ran on Swarming
6 used to run locally and were converted:
- chromium.fyi.json: ClangToTMac tester
- chromium.fyi.json: ClangToTMacASan tester
- chromium.mac.json: Mac10.6 Tests
- chromium.mac.json: Mac10.8 Tests
- chromium.mac.json: Mac10.9 Tests
- chromium.mac.json: Mac10.9 Tests (dbg)

Ran:
  ./manage.py --convert sandbox_mac_unittests

BUG=98637
TBR=cpu

Review URL: https://codereview.chromium.org/1218203002

Cr-Commit-Position: refs/heads/master@{#337057}
5 files changed