Adding second layer of the echo canceller 3 functionality.

webrtc/modules/audio_processing/aec3/echo_path_variability.h

+/*
+ *  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_ECHO_PATH_VARIABILITY_H_
+#define WEBRTC_MODULES_AUDIO_PROCESSING_AEC3_ECHO_PATH_VARIABILITY_H_
+
+namespace webrtc {
+
+struct EchoPathVariability {
+  EchoPathVariability(bool gain_change, bool delay_change)
+      : gain_change(gain_change), delay_change(delay_change) {}
+
+  bool AudioPathChanged() const { return gain_change || delay_change; }
+  bool gain_change;
+  bool delay_change;
+};
+
+}  // namespace webrtc
+
+#endif  // WEBRTC_MODULES_AUDIO_PROCESSING_AEC3_ECHO_PATH_VARIABILITY_H_