Compile rtc_base_objc and rtc_api_objc for Mac

webrtc/api/objc/RTCMediaConstraints.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 63 matching lines @@
   return rtc::scoped_ptr<webrtc::MediaConstraints>(nativeConstraints);
 }
 
 + (webrtc::MediaConstraintsInterface::Constraints)
     nativeConstraintsForConstraints:
         (NSDictionary<NSString *, NSString *> *)constraints {
   webrtc::MediaConstraintsInterface::Constraints nativeConstraints;
   for (NSString *key in constraints) {
     NSAssert([key isKindOfClass:[NSString class]],
              @"%@ is not an NSString.", key);
-    NSAssert([constraints[key] isKindOfClass:[NSString class]],
-             @"%@ is not an NSString.", constraints[key]);
+    NSAssert([[constraints objectForKey:key] isKindOfClass:[NSString class]],
+             @"%@ is not an NSString.", [constraints objectForKey:key]);
     nativeConstraints.push_back(webrtc::MediaConstraintsInterface::Constraint(
-        key.stdString, constraints[key].stdString));
+        key.stdString, [constraints objectForKey:key].stdString));
   }
   return nativeConstraints;
 }
 
 @end