Convert sandbox_mac_unittests to run exclusively on Swarming

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

Committed: https://crrev.com/92a101b6d732b8af7f12f19459e382181586373a
Cr-Commit-Position: refs/heads/master@{#337057}

[Nico, 2015-07-01 04:56:13]

thakis@chromium.org changed reviewers:
+ maruel@chromium.org, rsesek@chromium.org

[Nico, 2015-07-01 04:56:14]

rsesek: sandbox/
maruel: testing/

[M-A Ruel, 2015-07-01 14:33:31]

lgtm

[Robert Sesek, 2015-07-01 17:19:27]

lgtm

[Nico, 2015-07-01 17:30:06]

The CQ bit was checked by thakis@chromium.org

[commit-bot: I haz the power, 2015-07-01 17:31:04]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1218203002/1

[commit-bot: I haz the power, 2015-07-01 17:40:59]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-07-01 17:41:00]

Try jobs failed on following builders:
  chromium_presubmit on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/chromium_presub...)

[Nico, 2015-07-01 17:48:43]

thakis@chromium.org changed reviewers:
+ cpu@chromium.org

[Nico, 2015-07-01 17:48:44]

tbr cpu for sandbox/owners (the isolate format forces us to put the isolate file
into sandbox/, not sandbox/mac. Since it's next to the linux isolate file, that
seems uncontroversial enough to tbr)

[Nico, 2015-07-01 17:48:48]

The CQ bit was checked by thakis@chromium.org

[commit-bot: I haz the power, 2015-07-01 17:49:29]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1218203002/1

[commit-bot: I haz the power, 2015-07-01 17:56:22]

Committed patchset #1 (id:1)

[cpu_(ooo_6.6-7.5), 2015-07-01 17:56:52]

homer: ....
nico: thats good!
homer: .. lgtm, can I go now?

[commit-bot: I haz the power, 2015-07-01 17:58:13]

Patchset 1 (id:??) landed as
https://crrev.com/92a101b6d732b8af7f12f19459e382181586373a
Cr-Commit-Position: refs/heads/master@{#337057}