Compile rtc_api_objc on Mac.

webrtc/api/objc/RTCStatsReport.mm

 /*
  *  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.
  */
 
@@ skipping 32 matching lines @@
     NSUInteger capacity = nativeReport.values().size();
     NSMutableDictionary *values =
         [NSMutableDictionary dictionaryWithCapacity:capacity];
     for (auto const &valuePair : nativeReport.values()) {
       NSString *key = [NSString stringForStdString:
           valuePair.second->display_name()];
       NSString *value = [NSString stringForStdString:
           valuePair.second->ToString()];
 
       // Not expecting duplicate keys.
-      RTC_DCHECK(values[key]);
-
-      values[key] = value;
+      RTC_DCHECK(![values objectForKey:key]);
+      [values setObject:value forKey:key];
     }
     _values = values;
   }
   return self;
 }
 
 @end