settings: Add async system salt retrieval logic in DeviceOAuth2TokenServiceFactory

chromeos/dbus/fake_cryptohome_client.cc

Index: chromeos/dbus/fake_cryptohome_client.cc
diff --git a/chromeos/dbus/fake_cryptohome_client.cc b/chromeos/dbus/fake_cryptohome_client.cc
index d23be710ec178c6cd4d953657c1ebe43f241c40e..861a1c5730c4fa211351f6f66be27b5e39f0d748 100644
--- a/chromeos/dbus/fake_cryptohome_client.cc
+++ b/chromeos/dbus/fake_cryptohome_client.cc
@@ -17,6 +17,7 @@ FakeCryptohomeClient::FakeCryptohomeClient()
       async_call_id_(1),
       tpm_is_ready_counter_(0),
       unmount_result_(true),
+      system_salt_(GetStubSystemSalt()),
       locked_(false),
       weak_ptr_factory_(this) {}
 
@@ -80,7 +81,7 @@ void FakeCryptohomeClient::AsyncRemove(
 }
 
 bool FakeCryptohomeClient::GetSystemSalt(std::vector<uint8>* salt) {
-  *salt = GetStubSystemSalt();
+  *salt = system_salt_;
   return true;
 }