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

chrome/common/net/gaia/oauth2_access_token_fetcher.h

+// 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_FETCHER_H_
+#define CHROME_COMMON_NET_GAIA_OAUTH2_ACCESS_TOKEN_FETCHER_H_
+#pragma once
+
+#include <string>
+
+#include "base/gtest_prod_util.h"
+#include "base/memory/scoped_ptr.h"
+#include "chrome/common/net/gaia/oauth2_access_token_consumer.h"
+#include "content/public/common/url_fetcher.h"
+#include "content/public/common/url_fetcher_delegate.h"
+#include "googleurl/src/gurl.h"
+
+class OAuth2AccessTokenFetcherTest;
+
+namespace net {
+class URLRequestContextGetter;
+class URLRequestStatus;
+}
+
+// Abstracts the details to get OAuth2 access token token from
+// OAuth2 refresh token.
+//
+// This class should be used on a single thread, but it can be whichever thread
+// that you like.
+// Also, do not reuse the same instance. Once Start() is called, the instance
+// should not be reused.
+//
+// Usage:
+// * Create an instance with a consumer.
+// * Call Start()
+// * The consumer passed in the constructor will be called on the same
+//   thread Start was called with the results.
+//
+// This class can handle one request at a time. To parallelize requests,
+// create multiple instances.
+class OAuth2AccessTokenFetcher : public content::URLFetcherDelegate {
+ public:
+  OAuth2AccessTokenFetcher(OAuth2AccessTokenConsumer* consumer,
+                           net::URLRequestContextGetter* getter,
+                           const std::string& source);
+  virtual ~OAuth2AccessTokenFetcher();
+
+  void Start(const std::string& refresh_token);
+
+  void CancelRequest();
+
+  // Implementation of content::URLFetcherDelegate
+  virtual void OnURLFetchComplete(const content::URLFetcher* source) OVERRIDE;
+
+ private:
+  enum State {
+    INITIAL,
+    GET_ACCESS_TOKEN_STARTED,
+    GET_ACCESS_TOKEN_DONE,
+    ERROR_STATE,
+  };
+
+  // Helper methods for the flow.
+  void StartGetAccessToken();
+  void EndGetAccessToken(const content::URLFetcher* source);
+
+  // Helper mehtods for reporting back results.
+  void ReportSuccess(const std::string& access_token);
+  void ReportFailure(GoogleServiceAuthError error);
+
+  // Other helpers.
+  static GURL MakeGetAccessTokenUrl();
+  static std::string MakeGetAccessTokenBody(const std::string& refresh_token);
+  static bool ParseGetAccessTokenResponse(const content::URLFetcher* source,
+                                          std::string* access_token);
+
+  // State that is set during construction.
+  OAuth2AccessTokenConsumer* const consumer_;
+  net::URLRequestContextGetter* const getter_;
+  std::string source_;
+  State state_;
+
+  // While a fetch is in progress.
+  scoped_ptr<content::URLFetcher> fetcher_;
+  std::string refresh_token_;
+
+  friend class OAuth2AccessTokenFetcherTest;
+  FRIEND_TEST_ALL_PREFIXES(OAuth2AccessTokenFetcherTest,
+                           ParseGetAccessTokenResponse);
+
+  DISALLOW_COPY_AND_ASSIGN(OAuth2AccessTokenFetcher);
+};
+
+#endif  // CHROME_COMMON_NET_GAIA_OAUTH2_ACCESS_TOKEN_FETCHER_H_