settings: Add async system salt retrieval logic in DeviceOAuth2TokenServiceFactory

chromeos/dbus/fake_cryptohome_client.h

Index: chromeos/dbus/fake_cryptohome_client.h
diff --git a/chromeos/dbus/fake_cryptohome_client.h b/chromeos/dbus/fake_cryptohome_client.h
index 3c960e8f4ccb1f9dc46150dd8a0fe9878748fb27..450678bd003feb4d535c2d7fe6b91ae6abd95e00 100644
--- a/chromeos/dbus/fake_cryptohome_client.h
+++ b/chromeos/dbus/fake_cryptohome_client.h
@@ -162,6 +162,13 @@ class CHROMEOS_EXPORT FakeCryptohomeClient : public CryptohomeClient {
     unmount_result_= result;
   }
 
+  // Sets the system salt which will be returned from GetSystemSalt(). By
+  // default, GetSystemSalt() returns the value generated by
+  // GetStubSystemSalt().
+  void set_system_salt(const std::vector<uint8>& system_salt) {
+    system_salt_ = system_salt;
+  }
+
   // Returns the stub system salt as raw bytes. (not as a string encoded in the
   // format used by SystemSaltGetter::ConvertRawSaltToHexString()).
   static std::vector<uint8> GetStubSystemSalt();
@@ -181,6 +188,7 @@ class CHROMEOS_EXPORT FakeCryptohomeClient : public CryptohomeClient {
   AsyncCallStatusWithDataHandler async_call_status_data_handler_;
   int tpm_is_ready_counter_;
   bool unmount_result_;
+  std::vector<uint8> system_salt_;
 
   std::vector<WaitForServiceToBeAvailableCallback>
       pending_wait_for_service_to_be_available_callbacks_;