Remove WEBRTC_IOS from RTCPeerConnectionFactory public header.

webrtc/api/objc/RTCPeerConnectionFactory.mm

Index: webrtc/api/objc/RTCPeerConnectionFactory.mm
diff --git a/webrtc/api/objc/RTCPeerConnectionFactory.mm b/webrtc/api/objc/RTCPeerConnectionFactory.mm
index a7f9c594dc84535ff3a9d94d66132e2279a08880..e2d602123102f1ed670ebebaa59b6649e16da149 100644
--- a/webrtc/api/objc/RTCPeerConnectionFactory.mm
+++ b/webrtc/api/objc/RTCPeerConnectionFactory.mm
@@ -44,13 +44,16 @@
   return self;
 }
 
-#if defined(WEBRTC_IOS)
+
 - (RTCAVFoundationVideoSource *)avFoundationVideoSourceWithConstraints:
     (nullable RTCMediaConstraints *)constraints {
+#if defined(WEBRTC_IOS)
   return [[RTCAVFoundationVideoSource alloc] initWithFactory:self
                                                  constraints:constraints];
-}
+#else
+  return nil;
 #endif
+}
 
 - (RTCAudioTrack *)audioTrackWithTrackId:(NSString *)trackId {
   return [[RTCAudioTrack alloc] initWithFactory:self