Compile rtc_base_objc and rtc_api_objc for Mac

webrtc/api/objc/RTCMediaConstraints.mm

Index: webrtc/api/objc/RTCMediaConstraints.mm
diff --git a/webrtc/api/objc/RTCMediaConstraints.mm b/webrtc/api/objc/RTCMediaConstraints.mm
index a53a517747f7a2ebcf9fb8f71b36a7ae8f3379bd..4922b2f812f3666a0bd0db19377e6686ca6af3ac 100644
--- a/webrtc/api/objc/RTCMediaConstraints.mm
+++ b/webrtc/api/objc/RTCMediaConstraints.mm
@@ -81,10 +81,10 @@ MediaConstraints::GetOptional() const {
   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;
 }