fix(video-quality): make sure the LD stream is enabled even when requested resolution is lower

This should fix the case when camera is started with 1080p and LD simulcast stream's resolution is 270p but the requested resolution is 180p.

modules/RTC/TraceablePeerConnection.js

@@ -2267,12 +2267,20 @@ TraceablePeerConnection.prototype.setSenderVideoConstraint = function(frameHeigh
     }
 
     if (this.isSimulcastOn()) {
-        // Determine the encodings that need to stay enabled based on the
-        // new frameHeight provided.
-        const encodingsEnabledState
-            = this.tpcUtils.getLocalStreamHeightConstraints(localVideoTrack.track)
-                .map(height => height <= newHeight);
-
+        // Determine the encodings that need to stay enabled based on the new frameHeight provided.
+        const encodingsEnabledState = this.tpcUtils.getLocalStreamHeightConstraints(localVideoTrack.track)
+            .map(height => height <= newHeight);
+
+        // Always keep the LD stream enabled, specifically when the LD stream's resolution is higher than of the
+        // requested resolution. This can happen when camera is captured at resolutions higher than 720p but the
+        // requested resolution is 180. Since getParameters doesn't give us information about the resolutions
+        // of the simulcast encodings, we have to rely on our initial config for the simulcast streams.
+        const ldStreamIndex = this.tpcUtils.localStreamEncodingsConfig
+            .findIndex(layer => layer.scaleResolutionDownBy === 4.0);
+
+        if (newHeight > 0 && ldStreamIndex !== -1) {
+            encodingsEnabledState[ldStreamIndex] = true;
+        }
         for (const encoding in parameters.encodings) {
             if (parameters.encodings.hasOwnProperty(encoding)) {
                 parameters.encodings[encoding].active = encodingsEnabledState[encoding];