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-Original-Commit-Position: refs/heads/master@{#337057}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 92a101b6d732b8af7f12f19459e382181586373a

gn_migration.gypi

@@ -543,6 +543,7 @@
             '../sql/sql.gyp:sql_unittests_run',
             '../sync/sync.gyp:sync_unit_tests_run',
             '../third_party/cacheinvalidation/cacheinvalidation.gyp:cacheinvalidation_unittests_run',
+            '../tools/gn/gn.gyp:gn_unittests_run',
             '../ui/accessibility/accessibility.gyp:accessibility_unittests_run',
             '../ui/app_list/app_list.gyp:app_list_unittests_run',
             '../ui/events/events.gyp:events_unittests_run',
@@ -551,6 +552,21 @@
             '../url/url.gyp:url_unittests_run',
           ],
           'conditions': [
+            ['OS=="linux"', {
+              'dependencies': [
+                '../sandbox/sandbox.gyp:sandbox_linux_unittests_run',
+              ],
+            }],
+            ['OS=="mac"', {
+              'dependencies': [
+                '../sandbox/sandbox.gyp:sandbox_mac_unittests_run',
+              ],
+            }],
+            ['use_ash==1', {
+              'dependencies': [
+                '../ash/ash.gyp:ash_unittests_run',
+              ],
+            }],
             ['use_aura==1', {
               'dependencies': [
                 '../ui/aura/aura.gyp:aura_unittests_run',
@@ -564,16 +580,6 @@
             }],
           ],
         }],
-        ['test_isolation_mode!="noop" and use_ash==1', {
-          'dependencies': [
-            '../ash/ash.gyp:ash_unittests_run',
-          ],
-        }],
-        ['test_isolation_mode!="noop" and OS=="linux"', {
-          'dependencies': [
-            '../sandbox/sandbox.gyp:sandbox_linux_unittests_run',
-          ],
-        }],
         ['use_openssl==1', {
           'dependencies': [
             # TODO(GYP): All of these targets still need to be converted.