Mark ui_base_unittests target to be used on Swarming.

testing/buildbot/chromium.linux.json

 {
   "Android Tests": {
     "scripts": [
       {
         "name": "webview_licenses",
         "script": "webview_licenses.py"
       }
     ]
   },
   "Android Tests (dbg)": {
@@ skipping 75 matching lines @@
       {
         "test": "skia_unittests"
       },
       {
         "test": "sql_unittests"
       },
       {
         "test": "sync_unit_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "ui_base_unittests"
       },
       {
         "test": "unit_tests"
       }
     ]
   },
   "Linux Tests": {
     "gtest_tests": [
       {
@@ skipping 725 matching lines @@
       }
     ],
     "scripts": [
       {
         "name": "nacl_integration",
         "script": "nacl_integration.py"
       }
     ]
   }
 }