Convert url_unittests to run exclusively on Swarming

testing/buildbot/chromium.linux.json

Index: testing/buildbot/chromium.linux.json
diff --git a/testing/buildbot/chromium.linux.json b/testing/buildbot/chromium.linux.json
index 07a0ab00cd50bc97db501c198193883e773e276e..5a600ac951ee354ce1c01ebc1b1d0ddd7ee71f9f 100644
--- a/testing/buildbot/chromium.linux.json
+++ b/testing/buildbot/chromium.linux.json
@@ -184,6 +184,9 @@
         "test": "ui_base_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "url_unittests"
       }
     ]
@@ -682,6 +685,9 @@
         "test": "unit_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "url_unittests"
       },
       {
@@ -954,6 +960,9 @@
         "test": "unit_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "url_unittests"
       },
       {
@@ -1210,6 +1219,9 @@
         "test": "unit_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "url_unittests"
       },
       {