Convert ash_unittests to run on Swarming everywhere.

testing/buildbot/chromium.chromiumos.json

 {
   "Linux ChromiumOS GN": {
     "additional_compile_targets": [
       "accessibility_unittests",
       "app_list_unittests",
       "aura_unittests",
       "browser_tests",
       "cacheinvalidation_unittests",
       "chromedriver_unittests",
       "chromeos_unittests",
@@ skipping 110 matching lines @@
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "app_list_unittests"
       },
       {
         "test": "app_shell_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "ash_unittests"
       },
       {
         "test": "aura_unittests"
       },
       {
         "test": "base_unittests"
       },
       {
         "test": "cacheinvalidation_unittests"
@@ skipping 521 matching lines @@
       },
       {
         "test": "views_unittests"
       },
       {
         "test": "wm_unittests"
       }
     ]
   }
 }