Add empty residual echo detector.

webrtc/modules/audio_processing/echo_detector/echo_detector.h

+/*
+ *  Copyright (c) 2016 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_ECHO_DETECTOR_ECHO_DETECTOR_H_
+#define WEBRTC_MODULES_AUDIO_PROCESSING_ECHO_DETECTOR_ECHO_DETECTOR_H_
+
+#include <stddef.h>
+#include "webrtc/base/array_view.h"
+
+namespace webrtc {
+
+class EchoDetector {
+ public:
+  EchoDetector() {}
+  ~EchoDetector() {}
+
+  void BufferFarend(const rtc::ArrayView<const float>& farend);

[hlundin-webrtc, 2016/10/14 12:36:21]

The typical pattern is to pass the ArrayView by value, not by reference. That
is:
void BufferFarend(rtc::ArrayView<const float> farend);

This does not mean that all of the data is passed by value, though. 
(kwiberg@, keep me honest here...)

[kwiberg-webrtc, 2016/10/14 13:15:24]

That's right---the ArrayView itself just points to the data, and it's extremely
lightweight (it's literally just a data pointer and a length, with a bunch of
useful methods), so the rule of thumb is to just pass it by value always.

[ivoc, 2016/10/14 14:45:35]

Ok, done.

+
+  void Process(const rtc::ArrayView<const float>& nearend);

[hlundin-webrtc, 2016/10/14 12:36:21]

Same here.

[ivoc, 2016/10/14 14:45:35]

Done.

+
+  void Initialize(int sample_rate_hz);
+};
+
+}  // namespace webrtc
+
+#endif  // WEBRTC_MODULES_AUDIO_PROCESSING_ECHO_DETECTOR_ECHO_DETECTOR_H_