Update iOS AppRTCDemo to use level controller.

webrtc/sdk/objc/Framework/Headers/WebRTC/RTCMediaConstraints.h

 /*
  *  Copyright 2015 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #import <Foundation/Foundation.h>
 
 #import <WebRTC/RTCMacros.h>
 
 NS_ASSUME_NONNULL_BEGIN
 
 RTC_EXTERN NSString * const kRTCMediaConstraintsMinAspectRatio;
 RTC_EXTERN NSString * const kRTCMediaConstraintsMaxAspectRatio;
 RTC_EXTERN NSString * const kRTCMediaConstraintsMaxWidth;
 RTC_EXTERN NSString * const kRTCMediaConstraintsMinWidth;
 RTC_EXTERN NSString * const kRTCMediaConstraintsMaxHeight;
 RTC_EXTERN NSString * const kRTCMediaConstraintsMinHeight;
 RTC_EXTERN NSString * const kRTCMediaConstraintsMaxFrameRate;
 RTC_EXTERN NSString * const kRTCMediaConstraintsMinFrameRate;
+RTC_EXTERN NSString * const kRTCMediaConstraintsLevelControl;
+
+RTC_EXTERN NSString * const kRTCMediaConstraintsValueTrue;
+RTC_EXTERN NSString * const kRTCMediaConstraintsValueFalse;
 
 RTC_EXPORT
 @interface RTCMediaConstraints : NSObject
 
 - (instancetype)init NS_UNAVAILABLE;
 
 /** Initialize with mandatory and/or optional constraints. */
 - (instancetype)initWithMandatoryConstraints:
     (nullable NSDictionary<NSString *, NSString *> *)mandatory
                          optionalConstraints:
     (nullable NSDictionary<NSString *, NSString *> *)optional
     NS_DESIGNATED_INITIALIZER;
 
 @end
 
 NS_ASSUME_NONNULL_END