[Chromecast] Add a return value after NOTREACHED in tests.

chromecast/media/audio/BUILD.gn

 # Copyright 2015 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("audio") {
   sources = [
     "cast_audio_manager.cc",
     "cast_audio_manager.h",
     "cast_audio_manager_factory.cc",
     "cast_audio_manager_factory.h",
     "cast_audio_output_stream.cc",
     "cast_audio_output_stream.h",
   ]
 
   configs += [ "//chromecast:config" ]
 
   public_deps = [
     "//chromecast/public/media",
   ]
 
   deps = [
     "//media",
   ]
 }
 
 source_set("test_support") {
   testonly = true
 
-  sources = [
-    "cast_audio_output_stream_unittest.cc",
-  ]
-
   configs += [ "//chromecast:config" ]
 
   deps = [
     ":audio",
     "//testing/gtest",
   ]
 }