Signin/OAuth: Create an AccessTokenFetcher helper class

components/signin/core/browser/BUILD.gn

 # Copyright 2014 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.
 
 if (is_android) {
   import("//build/config/android/rules.gni")
 }
 
 static_library("browser") {
   sources = [
     "about_signin_internals.cc",
     "about_signin_internals.h",
+    "access_token_fetcher.cc",
+    "access_token_fetcher.h",
     "account_fetcher_service.cc",
     "account_fetcher_service.h",
     "account_info.cc",
     "account_info.h",
     "account_info_fetcher.cc",
     "account_info_fetcher.h",
     "account_investigator.cc",
     "account_investigator.h",
     "account_reconcilor.cc",
     "account_reconcilor.h",
@@ skipping 116 matching lines @@
     "//components/webdata/common",
     "//google_apis:test_support",
     "//net:test_support",
     "//testing/gtest",
   ]
 }
 
 source_set("unit_tests") {
   testonly = true
   sources = [
+    "access_token_fetcher_unittest.cc",
     "account_info_unittest.cc",
     "account_investigator_unittest.cc",
     "account_tracker_service_unittest.cc",
     "gaia_cookie_manager_service_unittest.cc",
     "refresh_token_annotation_request_unittest.cc",
     "signin_error_controller_unittest.cc",
     "signin_header_helper_unittest.cc",
     "signin_investigator_unittest.cc",
     "signin_status_metrics_provider_unittest.cc",
     "webdata/token_service_table_unittest.cc",
   ]
 
   deps = [
     ":test_support",
+    "//base/test:test_support",
     "//components/content_settings/core/browser",
     "//components/os_crypt:test_support",
     "//components/pref_registry:pref_registry",
     "//components/signin/core/common",
     "//components/sync_preferences:test_support",
     "//testing/gmock",
   ]
 
   if (is_chromeos) {
     sources -= [
       "account_investigator_unittest.cc",
       "signin_status_metrics_provider_unittest.cc",
     ]
   }
 }
 
 if (is_android) {
   java_cpp_enum("investigated_scenario_java") {
     sources = [
       "signin_investigator.h",
     ]
   }
 }