Fix Chromium style checker warnings for MockAudioDecoder

webrtc/test/mock_audio_decoder.cc

+/*
+ *  Copyright (c) 2017 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.
+ */
+
+#include "webrtc/test/mock_audio_decoder.h"
+
+namespace webrtc {
+
+MockAudioDecoder::MockAudioDecoder() = default;
+MockAudioDecoder::~MockAudioDecoder() {
+  Die();
+}
+
+}  // namespace webrtc