Part 1 of work to do user sign in based on OAuth2:

chrome/common/net/gaia/oauth2_access_token_consumer.h

Index: chrome/common/net/gaia/oauth2_access_token_consumer.h
===================================================================
--- chrome/common/net/gaia/oauth2_access_token_consumer.h	(revision 0)
+++ chrome/common/net/gaia/oauth2_access_token_consumer.h	(revision 0)
@@ -0,0 +1,23 @@
+// Copyright (c) 2011 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.
+
+#ifndef CHROME_COMMON_NET_GAIA_OAUTH2_ACCESS_TOKEN_CONSUMER_H_
+#define CHROME_COMMON_NET_GAIA_OAUTH2_ACCESS_TOKEN_CONSUMER_H_
+#pragma once
+
+#include <string>
+
+class GoogleServiceAuthError;
+
+// An interface that defines the callbacks for consumers to which
+// OAuth2AccessTokenFetcher can return results.
+class OAuth2AccessTokenConsumer {
+ public:
+  virtual ~OAuth2AccessTokenConsumer() {}
+
+  virtual void OnGetTokenSuccess(const std::string& access_token) {}
+  virtual void OnGetTokenFailure(const GoogleServiceAuthError& error) {}
+};
+
+#endif  // CHROME_COMMON_NET_GAIA_OAUTH2_ACCESS_TOKEN_CONSUMER_H_