Convert mojo_common_unittests to run exclusively on Swarming

testing/buildbot/chromium.win.json

Index: testing/buildbot/chromium.win.json
diff --git a/testing/buildbot/chromium.win.json b/testing/buildbot/chromium.win.json
index 51f6c7e9c914b03225d633a55cf1e0f93b567a0c..c2228035e4379d7d0539a21088178166c7299fac 100644
--- a/testing/buildbot/chromium.win.json
+++ b/testing/buildbot/chromium.win.json
@@ -172,6 +172,9 @@
         "test": "midi_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_common_unittests"
       },
       {
@@ -500,6 +503,9 @@
         "test": "midi_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_common_unittests"
       },
       {
@@ -951,6 +957,9 @@
         "test": "midi_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_common_unittests"
       },
       {
@@ -1259,6 +1268,9 @@
         "test": "midi_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_common_unittests"
       },
       {