Convert Mojo tests to use swarming.

testing/buildbot/chromium.mac.json

Index: testing/buildbot/chromium.mac.json
diff --git a/testing/buildbot/chromium.mac.json b/testing/buildbot/chromium.mac.json
index fa4afa673f8b184c4d66cd66bb79414b7be50ff6..2dbd81ab3812b22851be0f1d2f30ba5e8aacaa66 100644
--- a/testing/buildbot/chromium.mac.json
+++ b/testing/buildbot/chromium.mac.json
@@ -276,6 +276,9 @@
         "test": "mojo_public_system_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_system_unittests"
       },
       {
@@ -598,6 +601,9 @@
         "test": "mojo_public_system_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_system_unittests"
       },
       {
@@ -920,6 +926,9 @@
         "test": "mojo_public_system_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_system_unittests"
       },
       {
@@ -1243,6 +1252,9 @@
         "test": "mojo_public_system_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_system_unittests"
       },
       {