fix facingMode for mobile web. (#303)

lib/src/track/local/video.dart

@@ -206,10 +206,13 @@ extension LocalVideoTrackExt on LocalVideoTrack {
       logger.warning('Not a camera track');
       return;
     }
-
-    await restartTrack(
-      options.copyWith(cameraPosition: position),
-    );
+    final newOptions = CameraCaptureOptions(
+        cameraPosition: position,
+        deviceId: null,
+        maxFrameRate: options.maxFrameRate,
+        params: options.params);
+    await restartTrack(newOptions);
+    currentOptions = newOptions;
   }
 
   Future<void> switchCamera(String deviceId, {bool fastSwitch = false}) async {

lib/src/track/options.dart

@@ -44,8 +44,9 @@ class CameraCaptureOptions extends VideoCaptureOptions {
   Map<String, dynamic> toMediaConstraintsMap() {
     var constraints = <String, dynamic>{
       ...super.toMediaConstraintsMap(),
-      'facingMode':
-          cameraPosition == CameraPosition.front ? 'user' : 'environment',
+      if (deviceId == null)
+        'facingMode':
+            cameraPosition == CameraPosition.front ? 'user' : 'environment'
     };
     if (deviceId != null) {
       if (kIsWeb) {