--- a/dom/webidl/MediaStreamTrack.webidl
+++ b/dom/webidl/MediaStreamTrack.webidl
@@ -52,16 +52,17 @@ dictionary MediaTrackConstraintSet {
     ConstrainDOMString deviceId;
     ConstrainLong viewportOffsetX;
     ConstrainLong viewportOffsetY;
     ConstrainLong viewportWidth;
     ConstrainLong viewportHeight;
     ConstrainBoolean echoCancellation;
     ConstrainBoolean noiseSuppression;
     ConstrainBoolean autoGainControl;
+    ConstrainLong channelCount;
 
     // Deprecated with warnings:
     ConstrainBoolean mozNoiseSuppression;
     ConstrainBoolean mozAutoGainControl;
 };
 
 dictionary MediaTrackConstraints : MediaTrackConstraintSet {
     sequence<MediaTrackConstraintSet> advanced;

--- a/dom/webidl/MediaTrackSettings.webidl
+++ b/dom/webidl/MediaTrackSettings.webidl
@@ -11,16 +11,17 @@ dictionary MediaTrackSettings {
     long      width;
     long      height;
     double    frameRate;
     DOMString facingMode;
     DOMString deviceId;
     boolean echoCancellation;
     boolean noiseSuppression;
     boolean autoGainControl;
+    long      channelCount;
 
     // Mozilla-specific extensions:
 
     // http://fluffy.github.io/w3c-screen-share/#screen-based-video-constraints
     // OBE by http://w3c.github.io/mediacapture-screen-share
 
     DOMString mediaSource;
 

--- a/dom/webidl/MediaTrackSupportedConstraints.webidl
+++ b/dom/webidl/MediaTrackSupportedConstraints.webidl
@@ -15,16 +15,17 @@ dictionary MediaTrackSupportedConstraint
     boolean facingMode = true;
     boolean volume;             // to be supported
     boolean sampleRate;         // to be supported
     boolean sampleSize;         // to be supported
     boolean echoCancellation = true;
     boolean noiseSuppression = true;
     boolean autoGainControl = true;
     boolean latency;            // to be supported
+    boolean channelCount = true;
     boolean deviceId = true;
     boolean groupId;            // to be supported
 
     // Mozilla-specific extensions:
 
     // http://fluffy.github.io/w3c-screen-share/#screen-based-video-constraints
     // OBE by http://w3c.github.io/mediacapture-screen-share