Adding full initial version of delay estimation functionality in echo canceller 3

webrtc/modules/audio_processing/aec3/decimator_by_4.h

Index: webrtc/modules/audio_processing/aec3/decimator_by_4.h
diff --git a/webrtc/modules/audio_processing/aec3/decimator_by_4.h b/webrtc/modules/audio_processing/aec3/decimator_by_4.h
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+++ b/webrtc/modules/audio_processing/aec3/decimator_by_4.h
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+/*
+ *  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.
+ */
+
+#ifndef WEBRTC_MODULES_AUDIO_PROCESSING_AEC3_DECIMATOR_BY_4_H_
+#define WEBRTC_MODULES_AUDIO_PROCESSING_AEC3_DECIMATOR_BY_4_H_
+
+#include <array>
+
+#include "webrtc/base/array_view.h"
+#include "webrtc/base/constructormagic.h"
+#include "webrtc/modules/audio_processing/aec3/aec3_constants.h"
+#include "webrtc/modules/audio_processing/aec3/cascaded_biquad_filter.h"
+
+namespace webrtc {
+
+// Provides functionality for decimating a signal by 4.
+class DecimatorBy4 {
+ public:
+  DecimatorBy4();
+
+  // Downsamples the signal.
+  void Decimate(rtc::ArrayView<const float> in,
+                std::array<float, kSubBlockSize>* out);
+
+ private:
+  CascadedBiQuadFilter low_pass_filter_;
+
+  RTC_DISALLOW_COPY_AND_ASSIGN(DecimatorBy4);
+};
+}  // namespace webrtc
+
+#endif  // WEBRTC_MODULES_AUDIO_PROCESSING_AEC3_DECIMATOR_BY_4_H_