Compile rtc_base_objc and rtc_api_objc for Mac

webrtc/api/objc/RTCStatsReport.mm

Index: webrtc/api/objc/RTCStatsReport.mm
diff --git a/webrtc/api/objc/RTCStatsReport.mm b/webrtc/api/objc/RTCStatsReport.mm
index 35a52290145534431b73c1640233ef24f973c71c..31c804a9cfab166cf689f2b39a789d0328867960 100644
--- a/webrtc/api/objc/RTCStatsReport.mm
+++ b/webrtc/api/objc/RTCStatsReport.mm
@@ -50,9 +50,9 @@
           valuePair.second->ToString()];
 
       // Not expecting duplicate keys.
-      RTC_DCHECK(values[key]);
+      RTC_DCHECK([values objectForKey:key]);
 
-      values[key] = value;
+      [values setObject:value forKey:key];
     }
     _values = values;
   }