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

chrome/browser/chromeos/extensions/wallpaper_api.cc

Index: chrome/browser/chromeos/extensions/wallpaper_api.cc
diff --git a/chrome/browser/chromeos/extensions/wallpaper_api.cc b/chrome/browser/chromeos/extensions/wallpaper_api.cc
index 2ae5cc168247516db2af51a6c6828e18f8d97b27..f55a911ae0c848c66e025d58e29a7d7afb826e4e 100644
--- a/chrome/browser/chromeos/extensions/wallpaper_api.cc
+++ b/chrome/browser/chromeos/extensions/wallpaper_api.cc
@@ -93,7 +93,7 @@ base::LazyInstance<WallpaperFetcher> g_wallpaper_fetcher =
 
 }  // namespace
 
-WallpaperSetWallpaperFunction::WallpaperSetWallpaperFunction() {
+WallpaperSetWallpaperFunction::WallpaperSetWallpaperFunction() : user_id_(std::string(), std::string()) {

[Denis Kuznetsov (DE-MUC), 2015/06/10 16:50:45]

EmptyUserID() ?

 }
 
 WallpaperSetWallpaperFunction::~WallpaperSetWallpaperFunction() {
@@ -104,8 +104,8 @@ bool WallpaperSetWallpaperFunction::RunAsync() {
   params_ = set_wallpaper::Params::Create(*args_);
   EXTENSION_FUNCTION_VALIDATE(params_);
 
-  // Gets email address and username hash while at UI thread.
-  user_id_ = user_manager::UserManager::Get()->GetLoggedInUser()->email();
+  // Gets user id and username hash while at UI thread.
+  user_id_ = user_manager::UserManager::Get()->GetLoggedInUser()->GetUserID();
   user_id_hash_ =
       user_manager::UserManager::Get()->GetLoggedInUser()->username_hash();
 
@@ -142,7 +142,7 @@ void WallpaperSetWallpaperFunction::OnWallpaperDecoded(
   wallpaper::WallpaperLayout layout = wallpaper_api_util::GetLayoutEnum(
       extensions::api::wallpaper::ToString(params_->details.layout));
   bool update_wallpaper =
-      user_id_ == user_manager::UserManager::Get()->GetActiveUser()->email();
+      user_id_ == user_manager::UserManager::Get()->GetActiveUser()->GetUserID();
   wallpaper_manager->SetCustomWallpaper(user_id_,
                                         user_id_hash_,
                                         params_->details.filename,