Get response url from XMLHttpRequest

Summary:
An HTTP request may be redirected to another URL, sometimes we need to know the URL where the  response comes from.

If the server is in control, we can add an HTTP header X-Request-URL for the redirect URL. However there will be cases that 3rd party services are used.

This PR retrieves the response URL from native networking module and passes to it XMLHttpRequest. The fetch API built on XMLHttpRequest also benefits from this feature.
Closes https://github.com/facebook/react-native/pull/4981

Reviewed By: svcscm

Differential Revision: D2811392

Pulled By: lexs

fb-gh-sync-id: 3ec356fb92f8011b6a243d6879172877a3dc498a

ReactAndroid/src/main/java/com/facebook/react/modules/network/NetworkingModule.java

@@ -289,6 +289,7 @@ public final class NetworkingModule extends ReactContextBaseJavaModule {
     args.pushInt(requestId);
     args.pushInt(response.code());
     args.pushMap(headers);
+    args.pushString(response.request().urlString());
 
     getEventEmitter().emit("didReceiveNetworkResponse", args);
   }