Implement CryptAuthAccessTokenFetcher.

components/proximity_auth/cryptauth/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.
 
 source_set("cryptauth") {
   sources = [
     "cryptauth_access_token_fetcher.h",
+    "cryptauth_account_token_fetcher.cc",
+    "cryptauth_account_token_fetcher.h",
     "cryptauth_api_call_flow.cc",
     "cryptauth_api_call_flow.h",
     "cryptauth_client.cc",
     "cryptauth_client.h",
   ]
 
   deps = [
     "//base",
     "//google_apis",
     "//net",
   ]
 
   public_deps = [
     "proto",
   ]
 }
 
 source_set("unit_tests") {
   testonly = true
   sources = [
+    "cryptauth_account_token_fetcher_unittest.cc",
     "cryptauth_api_call_flow_unittest.cc",
     "cryptauth_client_unittest.cc",
   ]
 
   deps = [
     ":cryptauth",
     "//base/test:test_support",
+    "//google_apis:test_support",
     "//net:test_support",
     "//testing/gtest",
   ]
 }