Compile rtc_api_objc on Mac.

webrtc/api/objc/RTCMediaConstraints+Private.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 "RTCMediaConstraints.h"
 
 #include "webrtc/api/mediaconstraintsinterface.h"
 #include "webrtc/base/scoped_ptr.h"
 
+// TODO(hjon): Update nullability types. See http://crbug/webrtc/5592
+
 namespace webrtc {
 
 class MediaConstraints : public MediaConstraintsInterface {
  public:
   virtual ~MediaConstraints();
   MediaConstraints();
   MediaConstraints(
       const MediaConstraintsInterface::Constraints& mandatory,
       const MediaConstraintsInterface::Constraints& optional);
   virtual const Constraints& GetMandatory() const;
@@ skipping 13 matching lines @@
 
 /**
  * A MediaConstraints representation of this RTCMediaConstraints object. This is
  * needed to pass to the underlying C++ APIs.
  */
 - (rtc::scoped_ptr<webrtc::MediaConstraints>)nativeConstraints;
 
 /** Return a native Constraints object representing these constraints */
 + (webrtc::MediaConstraintsInterface::Constraints)
     nativeConstraintsForConstraints:
-        (NSDictionary<NSString *, NSString *> *)constraints;
+        (NSDictionary *)constraints;
+        // (NSDictionary<NSString *, NSString *> *)constraints;
 
 @end
 
 NS_ASSUME_NONNULL_END