Convert mojo_common_unittests to run exclusively on 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 645f82906fb065e4bd9607f84b893985cf24a83c..17a3e0ae9f721be2f54ea0fb2ff0f34c15f47aab 100644
--- a/testing/buildbot/chromium.mac.json
+++ b/testing/buildbot/chromium.mac.json
@@ -171,6 +171,9 @@
         "test": "midi_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_common_unittests"
       },
       {
@@ -410,6 +413,9 @@
         "test": "midi_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_common_unittests"
       },
       {
@@ -664,6 +670,9 @@
         "test": "midi_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_common_unittests"
       },
       {
@@ -918,6 +927,9 @@
         "test": "midi_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_common_unittests"
       },
       {
@@ -1173,6 +1185,9 @@
         "test": "midi_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_common_unittests"
       },
       {