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

chrome/common/net/gaia/oauth2_access_token_fetcher.cc

+// 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.
+
+#include "chrome/common/net/gaia/oauth2_access_token_fetcher.h"
+
+#include <algorithm>
+#include <string>
+
+#include "base/json/json_reader.h"
+#include "base/stringprintf.h"
+#include "base/values.h"
+#include "chrome/common/net/gaia/gaia_urls.h"
+#include "chrome/common/net/gaia/google_service_auth_error.h"
+#include "chrome/common/net/http_return.h"
+#include "net/base/escape.h"
+#include "net/base/load_flags.h"
+#include "net/url_request/url_request_context_getter.h"
+#include "net/url_request/url_request_status.h"
+
+using content::URLFetcher;
+using content::URLFetcherDelegate;
+using net::ResponseCookies;
+using net::URLRequestContextGetter;
+using net::URLRequestStatus;
+
+namespace {
+static const char kGetAccessTokenBodyFormat[] =
+    "client_id=%s&"
+    "client_secret=%s"
+    "grant_type=refresh_token&"
+    "refresh_token=%s";
+
+static const char kAccessTokenKey[] = "access_token";
+
+static bool GetStringFromDictionary(const DictionaryValue* dict,
+                                    const std::string& key,
+                                    std::string* value) {
+  Value* json_value;
+  if (!dict->Get(key, &json_value))
+    return false;
+  if (json_value->GetType() != base::Value::TYPE_STRING)
+    return false;
+
+  StringValue* json_str_value = static_cast<StringValue*>(json_value);
+  json_str_value->GetAsString(value);
+  return true;
+}
+
+static GoogleServiceAuthError CreateAuthError(URLRequestStatus status) {
+  CHECK(!status.is_success());
+  if (status.status() == URLRequestStatus::CANCELED) {
+    return GoogleServiceAuthError(GoogleServiceAuthError::REQUEST_CANCELED);
+  } else {
+    DLOG(WARNING) << "Could not reach Google Accounts servers: errno "
+                  << status.error();
+    return GoogleServiceAuthError::FromConnectionError(status.error());
+  }
+}
+
+static URLFetcher* CreateFetcher(URLRequestContextGetter* getter,
+                                 const GURL& url,
+                                 const std::string& body,
+                                 URLFetcherDelegate* delegate) {
+  bool empty_body = body.empty();
+  URLFetcher* result = URLFetcher::Create(
+      0, url,
+      empty_body ? URLFetcher::GET : URLFetcher::POST,
+      delegate);
+
+  result->SetRequestContext(getter);
+  result->SetLoadFlags(net::LOAD_DO_NOT_SEND_COOKIES |
+                       net::LOAD_DO_NOT_SAVE_COOKIES);
+
+  if (!empty_body)
+    result->SetUploadData("application/x-www-form-urlencoded", body);
+
+  return result;
+}
+}  // namespace
+
+OAuth2AccessTokenFetcher::OAuth2AccessTokenFetcher(
+    OAuth2AccessTokenConsumer* consumer,
+    URLRequestContextGetter* getter,
+    const std::string& source)
+    : consumer_(consumer),
+      getter_(getter),
+      source_(source),
+      state_(INITIAL) { }
+
+OAuth2AccessTokenFetcher::~OAuth2AccessTokenFetcher() { }
+
+void OAuth2AccessTokenFetcher::CancelRequest() {
+  fetcher_.reset();
+}
+
+void OAuth2AccessTokenFetcher::Start(const std::string& refresh_token) {
+  refresh_token_ = refresh_token;
+  StartGetAccessToken();
+}
+
+void OAuth2AccessTokenFetcher::StartGetAccessToken() {
+  CHECK_EQ(INITIAL, state_);
+  state_ = GET_ACCESS_TOKEN_STARTED;
+  fetcher_.reset(CreateFetcher(
+      getter_,
+      MakeGetAccessTokenUrl(),
+      MakeGetAccessTokenBody(refresh_token_),
+      this));
+  fetcher_->Start();  // OnURLFetchComplete will be called.
+}
+
+void OAuth2AccessTokenFetcher::EndGetAccessToken(const URLFetcher* source) {
+  CHECK_EQ(GET_ACCESS_TOKEN_STARTED, state_);
+  state_ = GET_ACCESS_TOKEN_DONE;
+
+  URLRequestStatus status = source->GetStatus();
+  if (!status.is_success()) {
+    ReportFailure(CreateAuthError(status));
+    return;
+  }
+
+  if (source->GetResponseCode() != RC_REQUEST_OK) {
+    ReportFailure(GoogleServiceAuthError(
+        GoogleServiceAuthError::INVALID_GAIA_CREDENTIALS));
+    return;
+  }
+
+  // The request was successfully fetched and it returned OK.
+  // Parse out the access token.
+  std::string access_token;
+  if (!ParseGetAccessTokenResponse(source, &access_token)) {
+    ReportFailure(GoogleServiceAuthError(
+        GoogleServiceAuthError::UNEXPECTED_RESPONSE));
+    return;
+  }
+
+  ReportSuccess(access_token);
+}
+
+void OAuth2AccessTokenFetcher::ReportSuccess(const std::string& access_token) {
+  consumer_->OnGetTokenSuccess(access_token);
+}
+
+void OAuth2AccessTokenFetcher::ReportFailure(GoogleServiceAuthError error) {
+  state_ = ERROR_STATE;
+  consumer_->OnGetTokenFailure(error);
+}
+
+void OAuth2AccessTokenFetcher::OnURLFetchComplete(const URLFetcher* source) {
+  CHECK(source);
+  CHECK(state_ == GET_ACCESS_TOKEN_STARTED);
+  EndGetAccessToken(source);
+}
+
+// static
+GURL OAuth2AccessTokenFetcher::MakeGetAccessTokenUrl() {
+  return GURL(GaiaUrls::GetInstance()->oauth2_token_url());
+}
+
+// static
+std::string OAuth2AccessTokenFetcher::MakeGetAccessTokenBody(
+    const std::string& refresh_token) {
+  return StringPrintf(
+      kGetAccessTokenBodyFormat,
+      net::EscapeUrlEncodedData(
+          GaiaUrls::GetInstance()->oauth2_chrome_client_id(), true).c_str(),
+      net::EscapeUrlEncodedData(
+          GaiaUrls::GetInstance()->oauth2_chrome_client_secret(),
+              true).c_str(),
+      net::EscapeUrlEncodedData(refresh_token, true).c_str());
+}
+
+// static
+bool OAuth2AccessTokenFetcher::ParseGetAccessTokenResponse(
+    const URLFetcher* source,
+    std::string* access_token) {
+  CHECK(source);
+  CHECK(access_token);
+  std::string data;
+  source->GetResponseAsString(&data);
+  base::JSONReader reader;
+  scoped_ptr<base::Value> value(reader.Read(data, false));
+  if (!value.get() || value->GetType() != base::Value::TYPE_DICTIONARY)
+    return false;
+
+  DictionaryValue* dict = static_cast<DictionaryValue*>(value.get());
+  return GetStringFromDictionary(dict, kAccessTokenKey, access_token);
+}