Convert url_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 f9d6453e26330d0b68df4f09b9b73f4ee756d6db..d9754dc787ba8179430b787acd0233a887bf5382 100644
--- a/testing/buildbot/chromium.mac.json
+++ b/testing/buildbot/chromium.mac.json
@@ -204,6 +204,9 @@
         "test": "ui_base_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "url_unittests"
       }
     ],
@@ -421,6 +424,9 @@
         "test": "unit_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "url_unittests"
       }
     ],
@@ -632,6 +638,9 @@
         "test": "unit_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "url_unittests"
       }
     ],
@@ -843,6 +852,9 @@
         "test": "unit_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "url_unittests"
       }
     ],
@@ -1055,6 +1067,9 @@
         "test": "unit_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "url_unittests"
       }
     ],