Update common_audio

Corresponds to upstream commit 524e9b043e7e86fd72353b987c9d5f6a1ebf83e1

Update notes:

 * Moved src/ to webrtc/ to easily diff against the third_party/webrtc
   in the chromium tree

 * ARM/NEON/MIPS support is not yet hooked up

 * Tests have not been copied

webrtc/common_audio/vad/vad_gmm.h

@@ -0,0 +1,39 @@
+/*
+ *  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.
+ */
+
+// Gaussian probability calculations internally used in vad_core.c.
+
+#ifndef WEBRTC_COMMON_AUDIO_VAD_VAD_GMM_H_
+#define WEBRTC_COMMON_AUDIO_VAD_VAD_GMM_H_
+
+#include "webrtc/typedefs.h"
+
+// Calculates the probability for |input|, given that |input| comes from a
+// normal distribution with mean and standard deviation (|mean|, |std|).
+//
+// Inputs:
+//      - input         : input sample in Q4.
+//      - mean          : mean input in the statistical model, Q7.
+//      - std           : standard deviation, Q7.
+//
+// Output:
+//
+//      - delta         : input used when updating the model, Q11.
+//                        |delta| = (|input| - |mean|) / |std|^2.
+//
+// Return:
+//   (probability for |input|) =
+//    1 / |std| * exp(-(|input| - |mean|)^2 / (2 * |std|^2));
+int32_t WebRtcVad_GaussianProbability(int16_t input,
+                                      int16_t mean,
+                                      int16_t std,
+                                      int16_t* delta);
+
+#endif  // WEBRTC_COMMON_AUDIO_VAD_VAD_GMM_H_