feat(video-quality): add iframe event and getter.

modules/API/API.js

         });
     }
 
+    /**
+     * Notify external application that the video quality setting has changed.
+     *
+     * @param {number} videoQuality - The video quality. The number represents the maximum height of the video streams.
+     * @returns {void}
+     */
+    notifyVideoQualityChanged(videoQuality: number) {
+        this._sendEvent({
+            name: 'video-quality-changed',
+            videoQuality
+        });
+    }
+
     /**
      * Notify external application (if API is enabled) that message was
      * received.

modules/API/external/external_api.js

     'video-conference-left': 'videoConferenceLeft',
     'video-availability-changed': 'videoAvailabilityChanged',
     'video-mute-status-changed': 'videoMuteStatusChanged',
+    'video-quality-changed': 'videoQualityChanged',
     'screen-sharing-status-changed': 'screenSharingStatusChanged',
     'dominant-speaker-changed': 'dominantSpeakerChanged',
     'subject-change': 'subjectChange',
                 changeParticipantNumber(this, -1);
                 delete this._participants[this._myUserID];
                 break;
+            case 'video-quality-changed':
+                this._videoQuality = data.videoQuality;
+                break;
             }
 
             const eventName = events[name];
         return getCurrentDevices(this._transport);
     }
 
+    /**
+     * Returns the current video quality setting.
+     *
+     * @returns {number}
+     */
+    getVideoQuality() {
+        return this._videoQuality;
+    }
+
     /**
      * Check if the audio is available.
      *

react/features/base/config/configWhitelist.js

     'testing',
     'useStunTurn',
     'useTurnUdp',
+    'videoQuality.persist',
     'webrtcIceTcpDisable',
     'webrtcIceUdpDisable'
 ].concat(extraConfigWhitelist);

react/features/video-quality/middleware.js

 import logger from './logger';
 import { getMinHeightForQualityLvlMap } from './selector';
 
+declare var APP: Object;
+
 /**
  * Implements the middleware of the feature video-quality.
  *
         if (changedConference || changedPreferredVideoQuality) {
             _setSenderVideoConstraint(conference, preferredVideoQuality);
         }
+
+        if (typeof APP !== 'undefined' && changedPreferredVideoQuality) {
+            APP.API.notifyVideoQualityChanged(preferredVideoQuality);
+        }
     });