Avoid clicks when muting/unmuting a voe::Channel.

webrtc/modules/utility/include/audio_frame_operations.h

Index: webrtc/modules/utility/include/audio_frame_operations.h
diff --git a/webrtc/modules/utility/include/audio_frame_operations.h b/webrtc/modules/utility/include/audio_frame_operations.h
index 1551d8689422f50760ca5e5bcc53027bcfc22473..e12e3e561be8d439fe5a59709149a875fc2b0509 100644
--- a/webrtc/modules/utility/include/audio_frame_operations.h
+++ b/webrtc/modules/utility/include/audio_frame_operations.h
@@ -45,8 +45,13 @@ class AudioFrameOperations {
   // not stereo.
   static void SwapStereoChannels(AudioFrame* frame);
 
-  // Zeros out the audio and sets |frame.energy| to zero.
-  static void Mute(AudioFrame& frame);
+  // Conditionally zero out contents of |frame| for implementing audio mute:
+  //  |previous_frame_muted| &&  |current_frame_muted| - Zero out whole frame.
+  //  |previous_frame_muted| && !|current_frame_muted| - Fade-in at frame start.
+  // !|previous_frame_muted| &&  |current_frame_muted| - Fade-out at frame end.
+  // !|previous_frame_muted| && !|current_frame_muted| - Leave frame untouched.
+  static void Mute(AudioFrame* frame, bool previous_frame_muted,
+                   bool current_frame_muted);
 
   static int Scale(float left, float right, AudioFrame& frame);