[Sync] Turn notifier::PushClient into an interface

jingle/notifier/listener/push_client_observer.cc

Index: jingle/notifier/listener/push_client_observer.cc
diff --git a/jingle/notifier/listener/push_client_observer.cc b/jingle/notifier/listener/push_client_observer.cc
new file mode 100644
index 0000000000000000000000000000000000000000..04a877b52c78ec897a02a97d3281007731bd965b
--- /dev/null
+++ b/jingle/notifier/listener/push_client_observer.cc
@@ -0,0 +1,11 @@
+// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "jingle/notifier/listener/push_client_observer.h"
+
+namespace notifier {
+
+PushClientObserver::~PushClientObserver() {}
+
+}  // namespace notifier