chromeos: Make system tray audio item observe CrasAudioHandler directly

ash/common/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.
 
 source_set("unittests") {
   testonly = true
   sources = [
     "accelerators/accelerator_table_unittest.cc",
     "system/date/date_view_unittest.cc",
     "system/update/tray_update_unittest.cc",
@@ skipping 10 matching lines @@
     "//ash/test:test_support_without_content",
     "//ui/base",
     "//ui/display",
     "//ui/keyboard",
     "//ui/views",
     "//ui/wm",
   ]
 
   if (is_chromeos) {
     sources += [
+      "system/chromeos/audio/tray_audio_unittest.cc",
       "system/chromeos/brightness/tray_brightness_unittest.cc",
       "system/chromeos/screen_security/screen_tray_item_unittest.cc",
       "system/chromeos/supervised/tray_supervised_user_unittest.cc",
     ]
     deps += [ "//ui/message_center" ]
   }
 }