Adding platform support check for NetworkObserver

net/base/network_change_notifier.h

Index: net/base/network_change_notifier.h
diff --git a/net/base/network_change_notifier.h b/net/base/network_change_notifier.h
index 4290331cddf259652a26253a3aa797a2fa69f011..0299cad328b863fc392f15b59ed8485507efacf3 100644
--- a/net/base/network_change_notifier.h
+++ b/net/base/network_change_notifier.h
@@ -361,6 +361,9 @@ class NET_EXPORT NetworkChangeNotifier {
   static void AddMaxBandwidthObserver(MaxBandwidthObserver* observer);
   static void AddNetworkObserver(NetworkObserver* observer);
 
+  // Returns true if platform supports notifications required for observer.
+  static bool IsNetworkObserverSupported();
+
   // Unregisters |observer| from receiving notifications.  This must be called
   // on the same thread on which AddObserver() was called.  Like AddObserver(),
   // this is safe to call if Create() has not been called (as long as it doesn't