[net/dns] Isolate DnsConfigWatcher from DnsConfigService.

net/base/network_change_notifier_linux.h

Index: net/base/network_change_notifier_linux.h
diff --git a/net/base/network_change_notifier_linux.h b/net/base/network_change_notifier_linux.h
index 3dbb814623b90b9ab8a26aa13d59b041c9d4b298..00b02c2cbdadec7868b6b83de8b27bef67b89921 100644
--- a/net/base/network_change_notifier_linux.h
+++ b/net/base/network_change_notifier_linux.h
@@ -38,6 +38,7 @@ class NET_EXPORT_PRIVATE NetworkChangeNotifierLinux
   // The thread used to listen for notifications.  This relays the notification
   // to the registered observers without posting back to the thread the object
   // was created on.
+  // Also used for DnsConfigWatcher which requires TYPE_IO message loop.
   scoped_ptr<Thread> notifier_thread_;
 
   DISALLOW_COPY_AND_ASSIGN(NetworkChangeNotifierLinux);