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

webrtc/voice_engine/channel.h

Index: webrtc/voice_engine/channel.h
diff --git a/webrtc/voice_engine/channel.h b/webrtc/voice_engine/channel.h
index 7d9a17fcf39c86b79fd33b8a9289cf6d9b00cc2a..bb816bfd3d4831a0a06544637db87a7eb137aaa1 100644
--- a/webrtc/voice_engine/channel.h
+++ b/webrtc/voice_engine/channel.h
@@ -269,8 +269,8 @@ class Channel
   // VoEVolumeControl
   int GetSpeechOutputLevel(uint32_t& level) const;
   int GetSpeechOutputLevelFullRange(uint32_t& level) const;
-  int SetMute(bool enable);
-  bool Mute() const;
+  int SetInputMute(bool enable);

[peah-webrtc, 2016/03/23 15:57:04]

What is motivates the name change to InputMute? Can there be an OutputMute? I
think that it should be fine to continue naming it Mute. To me this name-change
makes the purpose of the function less intuitive. A channel should only be muted
or not muted, I would not be able to tell how an InputMuted channel behaves.

[the sun, 2016/03/23 21:30:53]

The voe::Channel is duplex. SetMute() could mean to mute either the input or the
output. The functions on the VoEVolumeControl interface (to which this class is
tightly tied) calling these functions are called SetInputMute() and
GetInputMute().

[peah-webrtc, 2016/03/24 07:04:54]

Yes, I saw the other InputMute function but this change, or naming does not make
sense to me, in particular since there is no OutputMute. Most likely it is
because I don't yet know the voe code but could you please explain what the
difference between an input and output mute would be. And also, as the
voe::Channel is duplex, how you determine which is the input and which is the
output.

[the sun, 2016/03/24 09:33:15]

Input mute = mute the microphone
Output mute = mute the speaker

The reason I changed it was that I needed to look through the code to make sure
"mute" was only used for muting the input. Why would there be the distinction
between "input mute" in VoEVolume, and just "mute" in voe::Channel? I figure it
makes sense to be consistent.
The voe::Channel can be an input, and output or both at the same time. There's
no determining. Yes, we want to get rid of this behavior.

+  bool InputMute() const;
   int SetOutputVolumePan(float left, float right);
   int GetOutputVolumePan(float& left, float& right) const;
   int SetChannelOutputVolumeScaling(float scaling);
@@ -536,10 +536,11 @@ class Channel
   bool _externalMixing;
   bool _mixFileWithMicrophone;
   // VoEVolumeControl
-  bool _mute;
-  float _panLeft;
-  float _panRight;
-  float _outputGain;
+  bool input_mute_ GUARDED_BY(volume_settings_critsect_);
+  bool cached_input_mute_;  // Only accessed from PrepareEncodeAndSend().
+  float _panLeft GUARDED_BY(volume_settings_critsect_);
+  float _panRight GUARDED_BY(volume_settings_critsect_);
+  float _outputGain GUARDED_BY(volume_settings_critsect_);
   // VoeRTP_RTCP
   uint32_t _lastLocalTimeStamp;
   int8_t _lastPayloadType;