Convert midi_unittests to run exclusively on Swarming

testing/buildbot/chromium.chromiumos.json

Index: testing/buildbot/chromium.chromiumos.json
diff --git a/testing/buildbot/chromium.chromiumos.json b/testing/buildbot/chromium.chromiumos.json
index a97dea54a8775f31fecf7d812f4bd3cb3480feca..eec59b408d9decab021712d01212b34590119a55 100644
--- a/testing/buildbot/chromium.chromiumos.json
+++ b/testing/buildbot/chromium.chromiumos.json
@@ -250,6 +250,9 @@
         "test": "message_center_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "midi_unittests"
       },
       {
@@ -459,6 +462,9 @@
         "test": "message_center_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "midi_unittests"
       },
       {
@@ -664,6 +670,9 @@
         "test": "message_center_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "midi_unittests"
       },
       {