Remove dependency on rtc::Thread and rtc_base from audio_mixer_unittests.

webrtc/modules/audio_mixer/BUILD.gn

 # Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 import("../../webrtc.gni")
 
@@ skipping 72 matching lines @@
       "gain_change_calculator.h",
       "sine_wave_generator.cc",
       "sine_wave_generator.h",
     ]
     deps = [
       ":audio_frame_manipulator",
       ":audio_mixer_impl",
       "..:module_api",
       "../../api:audio_mixer_api",
       "../../audio/utility:audio_frame_operations",
-      "../../base:rtc_base",
       "../../base:rtc_base_approved",
+      "../../base:rtc_task_queue",
       "../../test:test_support",
       "//testing/gmock",
     ]
   }
 }