Moved XMPP notifier library from chrome/browser/sync to chrome/common....

chrome/browser/sync/notifier/communicator/product_info.cc

-// Copyright (c) 2009 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 <string>
-
-namespace notifier {
-std::string GetUserAgentString() {
-  return kXmppProductName;
-}
-
-std::string GetProductSignature() {
-  return kXmppProductName;
-}
-}  // namespace notifier