Update audio_processing module

Corresponds to upstream commit 524e9b043e7e86fd72353b987c9d5f6a1ebf83e1

Update notes:

 * Pull in third party license file

 * Replace .gypi files with BUILD.gn to keep track of what changes
   upstream

 * Bunch of new filse pulled in as dependencies

 * Won't build yet due to changes needed on top of these

webrtc/common_audio/fft4g.h

@@ -0,0 +1,25 @@
+/*
+ *  Copyright (c) 2011 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.
+ */
+
+#ifndef WEBRTC_COMMON_AUDIO_FFT4G_H_
+#define WEBRTC_COMMON_AUDIO_FFT4G_H_
+
+#if defined(__cplusplus)
+extern "C" {
+#endif
+
+// Refer to fft4g.c for documentation.
+void WebRtc_rdft(size_t n, int isgn, float *a, size_t *ip, float *w);
+
+#if defined(__cplusplus)
+}
+#endif
+
+#endif  // WEBRTC_COMMON_AUDIO_FFT4G_H_