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diff --git a/third_party/libwebrtc/audio/utility/channel_mixer.h b/third_party/libwebrtc/audio/utility/channel_mixer.h
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+/*
+ *  Copyright (c) 2019 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 AUDIO_UTILITY_CHANNEL_MIXER_H_
+#define AUDIO_UTILITY_CHANNEL_MIXER_H_
+
+#include <stddef.h>
+#include <stdint.h>
+
+#include <memory>
+#include <vector>
+
+#include "api/audio/audio_frame.h"
+#include "api/audio/channel_layout.h"
+
+namespace webrtc {
+
+// ChannelMixer is for converting audio between channel layouts.  The conversion
+// matrix is built upon construction and used during each Transform() call.  The
+// algorithm works by generating a conversion matrix mapping each output channel
+// to list of input channels.  The transform renders all of the output channels,
+// with each output channel rendered according to a weighted sum of the relevant
+// input channels as defined in the matrix.
+// This file is derived from Chromium's media/base/channel_mixer.h.
+class ChannelMixer {
+ public:
+  // To mix two channels into one and preserve loudness, we must apply
+  // (1 / sqrt(2)) gain to each.
+  static constexpr float kHalfPower = 0.707106781186547524401f;
+
+  ChannelMixer(ChannelLayout input_layout, ChannelLayout output_layout);
+  ~ChannelMixer();
+
+  // Transforms all input channels corresponding to the selected `input_layout`
+  // to the number of channels in the selected `output_layout`.
+  // Example usage (downmix from stereo to mono):
+  //
+  //   ChannelMixer mixer(CHANNEL_LAYOUT_STEREO, CHANNEL_LAYOUT_MONO);
+  //   AudioFrame frame;
+  //   frame.samples_per_channel_ = 160;
+  //   frame.num_channels_ = 2;
+  //   EXPECT_EQ(2u, frame.channels());
+  //   mixer.Transform(&frame);
+  //   EXPECT_EQ(1u, frame.channels());
+  //
+  void Transform(AudioFrame* frame);
+
+ private:
+  bool IsUpMixing() const { return output_channels_ > input_channels_; }
+
+  // Selected channel layouts.
+  const ChannelLayout input_layout_;
+  const ChannelLayout output_layout_;
+
+  // Channel counts for input and output.
+  const size_t input_channels_;
+  const size_t output_channels_;
+
+  // 2D matrix of output channels to input channels.
+  std::vector<std::vector<float> > matrix_;
+
+  // 1D array used as temporary storage during the transformation.
+  std::unique_ptr<int16_t[]> audio_vector_;
+
+  // Number of elements allocated for `audio_vector_`.
+  size_t audio_vector_size_ = 0;
+
+  // Optimization case for when we can simply remap the input channels to output
+  // channels, i.e., when all scaling factors in `matrix_` equals 1.0.
+  bool remapping_;
+
+  // Delete the copy constructor and assignment operator.
+  ChannelMixer(const ChannelMixer& other) = delete;
+  ChannelMixer& operator=(const ChannelMixer& other) = delete;
+};
+
+}  // namespace webrtc
+
+#endif  // AUDIO_UTILITY_CHANNEL_MIXER_H_