Remove AudioDecoder from the Filter heirarchy.

media/base/audio_decoder.cc

Index: media/base/audio_decoder.cc
diff --git a/media/base/audio_decoder.cc b/media/base/audio_decoder.cc
new file mode 100644
index 0000000000000000000000000000000000000000..939066078c4fec03c64ebb59b78c9c868ecc02e1
--- /dev/null
+++ b/media/base/audio_decoder.cc
@@ -0,0 +1,13 @@
+// Copyright (c) 2012 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.
+
+#include "media/base/audio_decoder.h"
+
+namespace media {
+
+AudioDecoder::AudioDecoder() {}
+
+AudioDecoder::~AudioDecoder() {}
+
+}  // namespace media