ref(TPC): add getLocalVideoTrack

...and remove duplication.

modules/RTC/TraceablePeerConnection.js

     return tracks;
 };
 
+/**
+ * Retrieves the local video track.
+ *
+ * @returns {JitsiLocalTrack|undefined} - local video track.
+ */
+TraceablePeerConnection.prototype.getLocalVideoTrack = function() {
+    return this.getLocalTracks(MediaType.VIDEO)[0];
+};
+
 /**
  * Checks whether or not this {@link TraceablePeerConnection} instance contains
  * any local tracks for given <tt>mediaType</tt>.
 
         return Promise.resolve();
     }
-    const localVideoTrack = Array.from(this.localTracks.values()).find(t => t.isVideoTrack());
+    const localVideoTrack = this.getLocalVideoTrack();
     const videoSender = this.findSenderByKind(MediaType.VIDEO);
 
     if (!videoSender) {
 TraceablePeerConnection.prototype.setMaxBitRate = function(localTrack = null) {
     if (!localTrack) {
         // eslint-disable-next-line no-param-reassign
-        localTrack = Array.from(this.localTracks.values()).find(t => t.isVideoTrack());
+        localTrack = this.getLocalVideoTrack();
 
         if (!localTrack) {
             return Promise.resolve();
     if (!newHeight) {
         return Promise.resolve();
     }
-    const localVideoTrack = Array.from(this.localTracks.values()).find(t => t.isVideoTrack());
+    const localVideoTrack = this.getLocalVideoTrack();
 
     if (!localVideoTrack || localVideoTrack.isMuted() || localVideoTrack.videoType !== VideoType.CAMERA) {
         return Promise.resolve();