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

chrome/browser/ui/webui/options/chromeos/user_image_source.h

Index: chrome/browser/ui/webui/options/chromeos/user_image_source.h
diff --git a/chrome/browser/ui/webui/options/chromeos/user_image_source.h b/chrome/browser/ui/webui/options/chromeos/user_image_source.h
index ad11d6ada99bf08677de41ef81aee63083e5e448..3f9bb844083b12fbf1382b451688a37733caf4b6 100644
--- a/chrome/browser/ui/webui/options/chromeos/user_image_source.h
+++ b/chrome/browser/ui/webui/options/chromeos/user_image_source.h
@@ -17,6 +17,10 @@ namespace base {
 class RefCountedMemory;
 }
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 namespace options {
 
@@ -36,8 +40,8 @@ class UserImageSource : public content::URLDataSource {
   std::string GetMimeType(const std::string& path) const override;
 
   // Returns PNG encoded image for user with specified email. If there's
-  // no user with such email, returns the first default image.
-  static base::RefCountedMemory* GetUserImage(const std::string& email,
+  // no user with such user id, returns the first default image.
+  static base::RefCountedMemory* GetUserImage(const user_manager::UserID& user_id,
                                               ui::ScaleFactor scale_factor);
 
  private: