Compile rtc_api_objc on Mac.

webrtc/api/objc/RTCMediaConstraints+Private.h

Index: webrtc/api/objc/RTCMediaConstraints+Private.h
diff --git a/webrtc/api/objc/RTCMediaConstraints+Private.h b/webrtc/api/objc/RTCMediaConstraints+Private.h
index fa582ecae597ca70188e36cbb2df30c0ad4847b3..84966d137a0337662788fe05c9005fe43b15d7b3 100644
--- a/webrtc/api/objc/RTCMediaConstraints+Private.h
+++ b/webrtc/api/objc/RTCMediaConstraints+Private.h
@@ -13,6 +13,8 @@
 #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 {
@@ -46,7 +48,8 @@ NS_ASSUME_NONNULL_BEGIN
 /** Return a native Constraints object representing these constraints */
 + (webrtc::MediaConstraintsInterface::Constraints)
     nativeConstraintsForConstraints:
-        (NSDictionary<NSString *, NSString *> *)constraints;
+        (NSDictionary *)constraints;
+        // (NSDictionary<NSString *, NSString *> *)constraints;
 
 @end