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

chrome/browser/chromeos/extensions/wallpaper_private_api.cc

Index: chrome/browser/chromeos/extensions/wallpaper_private_api.cc
diff --git a/chrome/browser/chromeos/extensions/wallpaper_private_api.cc b/chrome/browser/chromeos/extensions/wallpaper_private_api.cc
index cce297f29283114f187bf89dac4fa7a202fb5d0c..73b31a50c927d73666aa7d9df8af104cef57b5a4 100644
--- a/chrome/browser/chromeos/extensions/wallpaper_private_api.cc
+++ b/chrome/browser/chromeos/extensions/wallpaper_private_api.cc
@@ -35,6 +35,7 @@
 #include "chrome/grit/generated_resources.h"
 #include "chromeos/chromeos_switches.h"
 #include "components/user_manager/user.h"
+#include "components/user_manager/user_id.h"
 #include "components/user_manager/user_manager.h"
 #include "components/wallpaper/wallpaper_layout.h"
 #include "content/public/browser/browser_thread.h"
@@ -340,7 +341,7 @@ bool WallpaperPrivateGetSyncSettingFunction::RunSync() {
 }
 
 WallpaperPrivateSetWallpaperIfExistsFunction::
-    WallpaperPrivateSetWallpaperIfExistsFunction() {}
+    WallpaperPrivateSetWallpaperIfExistsFunction() : user_id_(std::string(), std::string()) {}

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

EmptyUserID() ?

 
 WallpaperPrivateSetWallpaperIfExistsFunction::
     ~WallpaperPrivateSetWallpaperIfExistsFunction() {}
@@ -349,9 +350,9 @@ bool WallpaperPrivateSetWallpaperIfExistsFunction::RunAsync() {
   params = set_wallpaper_if_exists::Params::Create(*args_);
   EXTENSION_FUNCTION_VALIDATE(params);
 
-  // Gets email address from caller, ensuring multiprofile compatibility.
+  // Gets User ID from caller, ensuring multiprofile compatibility.
   const user_manager::User* user = GetUserFromBrowserContext(browser_context());
-  user_id_ = user->email();
+  user_id_ = user->GetUserID();
 
   base::FilePath wallpaper_path;
   base::FilePath fallback_path;
@@ -424,7 +425,7 @@ void WallpaperPrivateSetWallpaperIfExistsFunction::OnWallpaperDecoded(
       wallpaper_base::ToString(params->layout));
 
   bool update_wallpaper =
-      user_id_ == user_manager::UserManager::Get()->GetActiveUser()->email();
+      user_id_ == user_manager::UserManager::Get()->GetActiveUser()->GetUserID();
   wallpaper_manager->SetWallpaperFromImageSkia(
       user_id_, image, layout, update_wallpaper);
   bool is_persistent = !user_manager::UserManager::Get()
@@ -450,7 +451,7 @@ void WallpaperPrivateSetWallpaperIfExistsFunction::OnFileNotExists(
   OnFailure(error);
 }
 
-WallpaperPrivateSetWallpaperFunction::WallpaperPrivateSetWallpaperFunction() {
+WallpaperPrivateSetWallpaperFunction::WallpaperPrivateSetWallpaperFunction() : user_id_(std::string(), std::string()) {

[Denis Kuznetsov (DE-MUC), 2015/06/10 16:50:46]

EmptyUserID() ?

 }
 
 WallpaperPrivateSetWallpaperFunction::~WallpaperPrivateSetWallpaperFunction() {
@@ -460,9 +461,9 @@ bool WallpaperPrivateSetWallpaperFunction::RunAsync() {
   params = set_wallpaper::Params::Create(*args_);
   EXTENSION_FUNCTION_VALIDATE(params);
 
-  // Gets email address from caller, ensuring multiprofile compatibility.
+  // Gets user ID address from caller, ensuring multiprofile compatibility.
   const user_manager::User* user = GetUserFromBrowserContext(browser_context());
-  user_id_ = user->email();
+  user_id_ = user->GetUserID();
 
   StartDecode(params->wallpaper);
 
@@ -534,7 +535,7 @@ void WallpaperPrivateSetWallpaperFunction::SetDecodedWallpaper(
       wallpaper_base::ToString(params->layout));
 
   bool update_wallpaper =
-      user_id_ == user_manager::UserManager::Get()->GetActiveUser()->email();
+      user_id_ == user_manager::UserManager::Get()->GetActiveUser()->GetUserID();
   wallpaper_manager->SetWallpaperFromImageSkia(
       user_id_, *image.get(), layout, update_wallpaper);
 
@@ -565,7 +566,7 @@ bool WallpaperPrivateResetWallpaperFunction::RunAsync() {
       chromeos::WallpaperManager::Get();
   user_manager::UserManager* user_manager = user_manager::UserManager::Get();
 
-  std::string user_id = user_manager->GetActiveUser()->email();
+  user_manager::UserID user_id = user_manager->GetActiveUser()->GetUserID();
   wallpaper_manager->RemoveUserWallpaperInfo(user_id);
 
   wallpaper::WallpaperInfo info = {std::string(),
@@ -587,7 +588,7 @@ bool WallpaperPrivateResetWallpaperFunction::RunAsync() {
 }
 
 WallpaperPrivateSetCustomWallpaperFunction::
-    WallpaperPrivateSetCustomWallpaperFunction() {}
+    WallpaperPrivateSetCustomWallpaperFunction() : user_id_(std::string(), std::string()) {}

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

EmptyUserID() ?

 
 WallpaperPrivateSetCustomWallpaperFunction::
     ~WallpaperPrivateSetCustomWallpaperFunction() {}
@@ -596,9 +597,9 @@ bool WallpaperPrivateSetCustomWallpaperFunction::RunAsync() {
   params = set_custom_wallpaper::Params::Create(*args_);
   EXTENSION_FUNCTION_VALIDATE(params);
 
-  // Gets email address from caller, ensuring multiprofile compatibility.
+  // Gets User ID from caller, ensuring multiprofile compatibility.
   const user_manager::User* user = GetUserFromBrowserContext(browser_context());
-  user_id_ = user->email();
+  user_id_ = user->GetUserID();
   user_id_hash_ = user->username_hash();
 
   StartDecode(params->wallpaper);
@@ -624,7 +625,7 @@ void WallpaperPrivateSetCustomWallpaperFunction::OnWallpaperDecoded(
       wallpaper_base::ToString(params->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->file_name,
@@ -705,11 +706,11 @@ bool WallpaperPrivateSetCustomWallpaperLayoutFunction::RunAsync() {
   info.layout = wallpaper_api_util::GetLayoutEnum(
       wallpaper_base::ToString(params->layout));
 
-  std::string email =
-      user_manager::UserManager::Get()->GetActiveUser()->email();
+  const user_manager::UserID user_id =
+      user_manager::UserManager::Get()->GetActiveUser()->GetUserID();
   bool is_persistent = !user_manager::UserManager::Get()
                             ->IsCurrentUserNonCryptohomeDataEphemeral();
-  wallpaper_manager->SetUserWallpaperInfo(email, info, is_persistent);
+  wallpaper_manager->SetUserWallpaperInfo(user_id, info, is_persistent);
   wallpaper_manager->UpdateWallpaper(false /* clear_cache */);
   SendResponse(true);
 
@@ -757,8 +758,6 @@ bool WallpaperPrivateGetThumbnailFunction::RunAsync() {
   EXTENSION_FUNCTION_VALIDATE(params);
 
   base::FilePath thumbnail_path;
-  std::string email =
-      user_manager::UserManager::Get()->GetActiveUser()->email();
   if (params->source == wallpaper_private::WALLPAPER_SOURCE_ONLINE) {
     std::string file_name = GURL(params->url_or_file).ExtractFileName();
     CHECK(PathService::Get(chrome::DIR_CHROMEOS_WALLPAPER_THUMBNAILS,