ref(video-quality): handle promise returned by video quality methods

modules/xmpp/JingleSessionPC.js

      * Sets the maximum bitrates on the local video track if the current
      * session is a JVB session. Bitrate values from videoQuality settings
      * in config.js will be used for configuring the sender.
-     * @returns {void}
+     * @returns {Promise<void>} promise that will be resolved when the operation is
+     * successful and rejected otherwise.
      */
     setSenderMaxBitrates() {
         if (this._assertNotEnded() && !this.isP2P) {
             return this.peerconnection.setMaxBitRate();
         }
+
+        return Promise.resolve();
     }
 
     /**
     /**
      * Sets the degradation preference on the video sender. This setting determines if
      * resolution or framerate will be preferred when bandwidth or cpu is constrained.
-     * @returns {void}
+     * @returns {Promise<void>} promise that will be resolved when the operation is
+     * successful and rejected otherwise.
      */
     setSenderVideoDegradationPreference() {
         if (this._assertNotEnded()) {
-            this.peerconnection.setSenderVideoDegradationPreference();
+            return this.peerconnection.setSenderVideoDegradationPreference();
         }
+
+        return Promise.resolve();
     }
 
     /**
                 // Apply the video constraints, max bitrates and degradation preference on
                 // the video sender if needed.
                 if (track.isVideoTrack() && browser.doesVideoMuteByStreamRemove()) {
-                    this.setSenderMaxBitrates();
-                    this.setSenderVideoDegradationPreference();
-                    this.setSenderVideoConstraint();
+                    return this.setSenderMaxBitrates()
+                        .then(() => this.setSenderVideoDegradationPreference())
+                        .then(() => this.setSenderVideoConstraint());
                 }
             });
     }