Implement CryptAuthAccessTokenFetcher.

components/proximity_auth/cryptauth/BUILD.gn

Index: components/proximity_auth/cryptauth/BUILD.gn
diff --git a/components/proximity_auth/cryptauth/BUILD.gn b/components/proximity_auth/cryptauth/BUILD.gn
index 6b1fe35c15a355fb37ec8c56840dabd87e4d0f23..e35fcb6d395fdce7339bce5958424463c344acf4 100644
--- a/components/proximity_auth/cryptauth/BUILD.gn
+++ b/components/proximity_auth/cryptauth/BUILD.gn
@@ -5,6 +5,8 @@
 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",
@@ -25,6 +27,7 @@ source_set("cryptauth") {
 source_set("unit_tests") {
   testonly = true
   sources = [
+    "cryptauth_account_token_fetcher_unittest.cc",
     "cryptauth_api_call_flow_unittest.cc",
     "cryptauth_client_unittest.cc",
   ]
@@ -32,6 +35,7 @@ source_set("unit_tests") {
   deps = [
     ":cryptauth",
     "//base/test:test_support",
+    "//google_apis:test_support",
     "//net:test_support",
     "//testing/gtest",
   ]