Adds target for ash_mus_unittests filter file

testing/buildbot/filters/BUILD.gn

 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # This BUILD.gn file groups together filter files for particular test suites -
 # for example filters applicable to content_browsertests are exposed as
 # the //testing/buildbot/filters:content_browsertests target.
 
 # Note that all filter files (for a given test suite) are unconditionally listed
 # together, even if some filter files are applicable only to some platforms.
@@ skipping 26 matching lines @@
   ]
 }
 
 source_set("interactive_ui_tests_filters") {
   testonly = true
 
   data = [
     "//testing/buildbot/filters/site-per-process.interactive_ui_tests.filter",
   ]
 }
+
+source_set("ash_unittests_filters") {
+  testonly = true
+
+  data = [
+    "//testing/buildbot/filters/ash_mus_unittests.filter",
+  ]
+}