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

chrome/browser/chromeos/accessibility/accessibility_manager.h

Index: chrome/browser/chromeos/accessibility/accessibility_manager.h
diff --git a/chrome/browser/chromeos/accessibility/accessibility_manager.h b/chrome/browser/chromeos/accessibility/accessibility_manager.h
index 9f13b71626bea715a6a4e4d3be5d437405eb5fcf..01410116918918c8f9a8f799539f3d3b4d117943 100644
--- a/chrome/browser/chromeos/accessibility/accessibility_manager.h
+++ b/chrome/browser/chromeos/accessibility/accessibility_manager.h
@@ -28,6 +28,10 @@ class RenderViewHost;
 }
 class Profile;
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 enum AccessibilityNotificationType {
@@ -162,7 +166,7 @@ class AccessibilityManager
   bool IsBrailleDisplayConnected() const;
 
   // SessionStateObserver overrides:
-  void ActiveUserChanged(const std::string& user_id) override;
+  void ActiveUserChanged(const user_manager::UserID& user_id) override;
 
   void SetProfileForTest(Profile* profile);