Update API for Objective-C RTCMediaConstraints.

webrtc/api/objc/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>
 
-#include <string>
-
 NS_ASSUME_NONNULL_BEGIN
 
-@interface NSString (StdString)
+@interface RTCMediaConstraints : NSObject
 
-@property(nonatomic, readonly) std::string stdString;
+- (instancetype)init NS_UNAVAILABLE;
 
-+ (std::string)stdStringForString:(NSString *)nsString;
-+ (NSString *)stringForStdString:(const std::string&)stdString;
+/** 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