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/window_generator.cc

@@ -0,0 +1,72 @@
+/*
+ *  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.
+ */
+
+#define _USE_MATH_DEFINES
+
+#include "webrtc/common_audio/window_generator.h"
+
+#include <cmath>
+#include <complex>
+
+#include "webrtc/base/checks.h"
+
+using std::complex;
+
+namespace {
+
+// Modified Bessel function of order 0 for complex inputs.
+complex<float> I0(complex<float> x) {
+  complex<float> y = x / 3.75f;
+  y *= y;
+  return 1.0f + y * (
+    3.5156229f + y * (
+      3.0899424f + y * (
+        1.2067492f + y * (
+          0.2659732f + y * (
+            0.360768e-1f + y * 0.45813e-2f)))));
+}
+
+}  // namespace
+
+namespace webrtc {
+
+void WindowGenerator::Hanning(int length, float* window) {
+  RTC_CHECK_GT(length, 1);
+  RTC_CHECK(window != nullptr);
+  for (int i = 0; i < length; ++i) {
+    window[i] = 0.5f * (1 - cosf(2 * static_cast<float>(M_PI) * i /
+                                 (length - 1)));
+  }
+}
+
+void WindowGenerator::KaiserBesselDerived(float alpha, size_t length,
+                                          float* window) {
+  RTC_CHECK_GT(length, 1U);
+  RTC_CHECK(window != nullptr);
+
+  const size_t half = (length + 1) / 2;
+  float sum = 0.0f;
+
+  for (size_t i = 0; i <= half; ++i) {
+    complex<float> r = (4.0f * i) / length - 1.0f;
+    sum += I0(static_cast<float>(M_PI) * alpha * sqrt(1.0f - r * r)).real();
+    window[i] = sum;
+  }
+  for (size_t i = length - 1; i >= half; --i) {
+    window[length - i - 1] = sqrtf(window[length - i - 1] / sum);
+    window[i] = window[length - i - 1];
+  }
+  if (length % 2 == 1) {
+    window[half - 1] = sqrtf(window[half - 1] / sum);
+  }
+}
+
+}  // namespace webrtc
+