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

chrome/browser/chromeos/extensions/wallpaper_private_api.h

Index: chrome/browser/chromeos/extensions/wallpaper_private_api.h
diff --git a/chrome/browser/chromeos/extensions/wallpaper_private_api.h b/chrome/browser/chromeos/extensions/wallpaper_private_api.h
index 71cfcba168b62888cb2bef7626f0303fb2bea359..623fb2129a1ccca28e7261f582735c4b418a4b80 100644
--- a/chrome/browser/chromeos/extensions/wallpaper_private_api.h
+++ b/chrome/browser/chromeos/extensions/wallpaper_private_api.h
@@ -11,6 +11,7 @@
 #include "base/threading/sequenced_worker_pool.h"
 #include "chrome/browser/chromeos/extensions/wallpaper_function_base.h"
 #include "chrome/common/extensions/api/wallpaper_private.h"
+#include "components/user_manager/user_id.h"
 #include "net/url_request/url_fetcher_delegate.h"
 
 namespace chromeos {
@@ -72,7 +73,7 @@ class WallpaperPrivateSetWallpaperIfExistsFunction
       params;
 
   // User id of the active user when this api is been called.
-  std::string user_id_;
+  user_manager::UserID user_id_;
 
   // Sequence token associated with wallpaper operations. Shared with
   // WallpaperManager.
@@ -108,7 +109,7 @@ class WallpaperPrivateSetWallpaperFunction : public WallpaperFunctionBase {
   gfx::ImageSkia wallpaper_;
 
   // User id of the active user when this api is been called.
-  std::string user_id_;
+  user_manager::UserID user_id_;
 
   // Sequence token associated with wallpaper operations. Shared with
   // WallpaperManager.
@@ -159,7 +160,7 @@ class WallpaperPrivateSetCustomWallpaperFunction
       params;
 
   // User id of the active user when this api is been called.
-  std::string user_id_;
+  user_manager::UserID user_id_;
 
   // User id hash of the logged in user.
   std::string user_id_hash_;