Signin/OAuth: Create an AccessTokenFetcher helper class

components/signin/core/browser/BUILD.gn

Index: components/signin/core/browser/BUILD.gn
diff --git a/components/signin/core/browser/BUILD.gn b/components/signin/core/browser/BUILD.gn
index a381719d0ba2a336d0b51ef16b1a47f2aaaf9460..e7d8bead1bcc79b2dac4f68ea238c5bf2912b444 100644
--- a/components/signin/core/browser/BUILD.gn
+++ b/components/signin/core/browser/BUILD.gn
@@ -10,6 +10,8 @@ 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",
@@ -146,6 +148,7 @@ static_library("test_support") {
 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",
@@ -160,6 +163,7 @@ source_set("unit_tests") {
 
   deps = [
     ":test_support",
+    "//base/test:test_support",
     "//components/content_settings/core/browser",
     "//components/os_crypt:test_support",
     "//components/pref_registry:pref_registry",