Convert url_unittests to run exclusively on Swarming

testing/buildbot/chromium.webkit.json

 {
   "Linux ChromiumOS Tests (1)": {
     "gtest_tests": [
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "accessibility_unittests"
       },
       {
@@ skipping 180 matching lines @@
         },
         "test": "ui_touch_selection_unittests"
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "unit_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "url_unittests"
       },
       {
         "test": "views_unittests"
       },
       {
         "test": "wm_unittests"
       }
     ]
   },
@@ skipping 191 matching lines @@
         },
         "test": "ui_touch_selection_unittests"
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "unit_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "url_unittests"
       },
       {
         "test": "views_unittests"
       },
       {
         "test": "wm_unittests"
       }
     ]
   },
@@ skipping 690 matching lines @@
         "name": "webkit_lint",
         "script": "webkit_lint.py"
       },
       {
         "name": "webkit_python_tests",
         "script": "webkit_python_tests.py"
       }
     ]
   }
 }