[net] Adds AddressTrackerLinux which keeps track of interface addresses using rtnetlink.

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 cc5ceff74ed9d392483b3e290d64d5c092805ff0..3021baf5e0a15c9adcc9b43647011d98f18c7106 100644
--- a/net/base/network_change_notifier_linux.h
+++ b/net/base/network_change_notifier_linux.h
@@ -18,6 +18,10 @@ class Bus;
 
 namespace net {
 
+namespace internal {
+class AddressTrackerLinux;
+}
+
 class NET_EXPORT_PRIVATE NetworkChangeNotifierLinux
     : public NetworkChangeNotifier {
  public:
@@ -27,6 +31,7 @@ class NET_EXPORT_PRIVATE NetworkChangeNotifierLinux
   static NetworkChangeNotifierLinux* CreateForTest(dbus::Bus* bus);
 
  private:
+  friend class internal::AddressTrackerLinux;
   class Thread;
 
   explicit NetworkChangeNotifierLinux(dbus::Bus* bus);