Convert sandbox_mac_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 bdeb859db710c2106344ccbb2fcf835e40be77b1..8000df44c1b6a6b479830456aa4f67e9f2b9b23a 100644
--- a/testing/buildbot/chromium.mac.json
+++ b/testing/buildbot/chromium.mac.json
@@ -439,6 +439,9 @@
         "test": "remoting_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "sandbox_mac_unittests"
       },
       {
@@ -680,6 +683,9 @@
         "test": "remoting_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "sandbox_mac_unittests"
       },
       {
@@ -921,6 +927,9 @@
         "test": "remoting_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "sandbox_mac_unittests"
       },
       {
@@ -1163,6 +1172,9 @@
         "test": "remoting_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "sandbox_mac_unittests"
       },
       {