We should use UserID object to identify users instead of username.

chrome/browser/chromeos/login/easy_unlock/easy_unlock_key_manager.h

Index: chrome/browser/chromeos/login/easy_unlock/easy_unlock_key_manager.h
diff --git a/chrome/browser/chromeos/login/easy_unlock/easy_unlock_key_manager.h b/chrome/browser/chromeos/login/easy_unlock/easy_unlock_key_manager.h
index 9d838753c09b2047c39b698f42303f3e2f24dd06..4f857bdc9f263739af61f0812464ee9fcd438f2c 100644
--- a/chrome/browser/chromeos/login/easy_unlock/easy_unlock_key_manager.h
+++ b/chrome/browser/chromeos/login/easy_unlock/easy_unlock_key_manager.h
@@ -22,6 +22,10 @@ class DictionaryValue;
 class ListValue;
 }
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 class UserContext;
@@ -54,7 +58,7 @@ class EasyUnlockKeyManager {
   // conversion fails (missing required propery). Note that
   // EasyUnlockDeviceKeyData contains a sub set of the remote device dictionary.
   static void DeviceDataToRemoteDeviceDictionary(
-      const std::string& user_id,
+      const user_manager::UserID& user_id,
       const EasyUnlockDeviceKeyData& data,
       base::DictionaryValue* dict);
   static bool RemoteDeviceDictionaryToDeviceData(
@@ -64,7 +68,7 @@ class EasyUnlockKeyManager {
   // Helpers to convert between EasyUnlockDeviceKeyDataList and remote devices
   // ListValue.
   static void DeviceDataListToRemoteDeviceList(
-      const std::string& user_id,
+      const user_manager::UserID& user_id,
       const EasyUnlockDeviceKeyDataList& data_list,
       base::ListValue* device_list);
   static bool RemoteDeviceListToDeviceDataList(