Add MediaTrackConstraintSet.{channelCount,latency}

I have not included `autoGainControl` or `noiseSuppression` in this
commit as they are only in the working draft, not the current candidate
recommendation.

Fixes microsoft#286.

Author: chris-morgan

baselines/dom.generated.d.ts

@@ -297,6 +297,8 @@ interface MediaTrackConstraints extends MediaTrackConstraintSet {
 
 interface MediaTrackConstraintSet {
     echoCancellation?: boolean | ConstrainBooleanParameters;
+    channelCount?: long | ConstrainLongRange;
+    latency?: double | ConstrainDoubleRange;
     aspectRatio?: number | ConstrainDoubleRange;
     deviceId?: string | string[] | ConstrainDOMStringParameters;
     facingMode?: string | string[] | ConstrainDOMStringParameters;

inputfiles/addedTypes.json

@@ -1770,5 +1770,17 @@
         "interface": "MediaTrackConstraintSet",
         "name": "echoCancellation?",
         "type": "boolean | ConstrainBooleanParameters"
+    },
+    {
+        "kind": "property",
+        "interface": "MediaTrackConstraintSet",
+        "name": "channelCount?",
+        "type": "long | ConstrainLongRange"
+    },
+    {
+        "kind": "property",
+        "interface": "MediaTrackConstraintSet",
+        "name": "latency?",
+        "type": "double | ConstrainDoubleRange"
     }
 ]