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

chrome/browser/chromeos/login/users/avatar/user_image_manager_impl.cc

Index: chrome/browser/chromeos/login/users/avatar/user_image_manager_impl.cc
diff --git a/chrome/browser/chromeos/login/users/avatar/user_image_manager_impl.cc b/chrome/browser/chromeos/login/users/avatar/user_image_manager_impl.cc
index 805423e21e6c2f82bfbf2e77581017e45b4e1a17..f282a141666c6670d6fa8e93675a765f6084088e 100644
--- a/chrome/browser/chromeos/login/users/avatar/user_image_manager_impl.cc
+++ b/chrome/browser/chromeos/login/users/avatar/user_image_manager_impl.cc
@@ -264,7 +264,7 @@ class UserImageManagerImpl::Job {
   // Notifies the |parent_| that the Job is done.
   void NotifyJobDone();
 
-  const std::string& user_id() const { return parent_->user_id(); }
+  const user_manager::UserID& user_id() const { return parent_->user_id(); }
 
   UserImageManagerImpl* parent_;
 
@@ -433,7 +433,7 @@ void UserImageManagerImpl::Job::UpdateUser() {
 void UserImageManagerImpl::Job::SaveImageAndUpdateLocalState() {
   base::FilePath user_data_dir;
   PathService::Get(chrome::DIR_USER_DATA, &user_data_dir);
-  image_path_ = user_data_dir.Append(user_id() + kSafeImagePathExtension);
+  image_path_ = user_data_dir.Append(user_id().GetUserEmail() + kSafeImagePathExtension);
 
   base::PostTaskAndReplyWithResult(
       parent_->background_task_runner_.get(),
@@ -461,7 +461,7 @@ void UserImageManagerImpl::Job::UpdateLocalState() {
     entry->Set(kImageURLNodeName, new base::StringValue(image_url_.spec()));
   DictionaryPrefUpdate update(g_browser_process->local_state(),
                               kUserImageProperties);
-  update->SetWithoutPathExpansion(user_id(), entry.release());
+  update->SetWithoutPathExpansion(user_id().GetUserEmail(), entry.release());
 
   parent_->user_manager_->NotifyLocalStateChanged();
 }
@@ -471,7 +471,7 @@ void UserImageManagerImpl::Job::NotifyJobDone() {
 }
 
 UserImageManagerImpl::UserImageManagerImpl(
-    const std::string& user_id,
+    const user_manager::UserID& user_id,
     user_manager::UserManager* user_manager)
     : UserImageManager(user_id),
       user_manager_(user_manager),
@@ -511,12 +511,12 @@ void UserImageManagerImpl::LoadUserImage() {
   const base::DictionaryValue* image_properties = NULL;
   if (prefs_images_unsafe) {
     needs_migration = prefs_images_unsafe->GetDictionaryWithoutPathExpansion(
-        user_id(), &image_properties);
+        user_id().GetUserEmail(), &image_properties);
     if (needs_migration)
       user_needs_migration_ = true;
   }
   if (prefs_images) {
-    prefs_images->GetDictionaryWithoutPathExpansion(user_id(),
+    prefs_images->GetDictionaryWithoutPathExpansion(user_id().GetUserEmail(),
                                                     &image_properties);
   }
 
@@ -588,7 +588,7 @@ void UserImageManagerImpl::UserLoggedIn(bool user_is_new,
           g_browser_process->local_state()->GetDictionary(kUserImages);
       const base::DictionaryValue* image_properties = NULL;
       if (prefs_images_unsafe->GetDictionaryWithoutPathExpansion(
-              user_id(), &image_properties)) {
+              user_id().GetUserEmail(), &image_properties)) {
         std::string image_path;
         image_properties->GetString(kImagePathNodeName, &image_path);
         job_.reset(new Job(this));
@@ -923,7 +923,7 @@ void UserImageManagerImpl::DeleteUserImageAndLocalStateEntry(
   DictionaryPrefUpdate update(g_browser_process->local_state(),
                               prefs_dict_root);
   const base::DictionaryValue* image_properties;
-  if (!update->GetDictionaryWithoutPathExpansion(user_id(), &image_properties))
+  if (!update->GetDictionaryWithoutPathExpansion(user_id().GetUserEmail(), &image_properties))
     return;
 
   std::string image_path;
@@ -935,7 +935,7 @@ void UserImageManagerImpl::DeleteUserImageAndLocalStateEntry(
                    base::FilePath(image_path),
                    false));
   }
-  update->RemoveWithoutPathExpansion(user_id(), NULL);
+  update->RemoveWithoutPathExpansion(user_id().GetUserEmail(), NULL);
 }
 
 void UserImageManagerImpl::OnJobChangedUserImage() {
@@ -963,7 +963,7 @@ void UserImageManagerImpl::OnJobDone() {
       g_browser_process->local_state()->GetDictionary(kUserImages);
   const base::DictionaryValue* image_properties = NULL;
   if (!prefs_images_unsafe->GetDictionaryWithoutPathExpansion(
-          user_id(), &image_properties)) {
+          user_id().GetUserEmail(), &image_properties)) {
     NOTREACHED();
     return;
   }
@@ -998,7 +998,7 @@ void UserImageManagerImpl::OnJobDone() {
 void UserImageManagerImpl::UpdateLocalStateAfterMigration() {
   DictionaryPrefUpdate update(g_browser_process->local_state(),
                               kUserImages);
-  update->RemoveWithoutPathExpansion(user_id(), NULL);
+  update->RemoveWithoutPathExpansion(user_id().GetUserEmail(), NULL);
 }
 
 void UserImageManagerImpl::TryToCreateImageSyncObserver() {