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

chrome/browser/ui/ash/multi_user/multi_user_window_manager_chromeos.cc

Index: chrome/browser/ui/ash/multi_user/multi_user_window_manager_chromeos.cc
diff --git a/chrome/browser/ui/ash/multi_user/multi_user_window_manager_chromeos.cc b/chrome/browser/ui/ash/multi_user/multi_user_window_manager_chromeos.cc
index 0535f8cbc5be840ba29a1206092fbc1840e2fa8e..c3175bae152ea9f63f170d86cea3539a82a1c6e2 100644
--- a/chrome/browser/ui/ash/multi_user/multi_user_window_manager_chromeos.cc
+++ b/chrome/browser/ui/ash/multi_user/multi_user_window_manager_chromeos.cc
@@ -30,6 +30,7 @@
 #include "chrome/browser/ui/browser_finder.h"
 #include "chrome/browser/ui/browser_list.h"
 #include "chrome/browser/ui/browser_window.h"
+#include "components/user_manager/user_id.h"
 #include "content/public/browser/notification_service.h"
 #include "extensions/browser/app_window/app_window.h"
 #include "extensions/browser/app_window/app_window_registry.h"
@@ -191,7 +192,7 @@ class AnimationSetter {
 // window observer will take care of that.
 class AppObserver : public extensions::AppWindowRegistry::Observer {
  public:
-  explicit AppObserver(const std::string& user_id) : user_id_(user_id) {}
+  explicit AppObserver(const user_manager::UserID& user_id) : user_id_(user_id) {}
   ~AppObserver() override {}
 
   // AppWindowRegistry::Observer overrides:
@@ -203,14 +204,15 @@ class AppObserver : public extensions::AppWindowRegistry::Observer {
   }
 
  private:
-  std::string user_id_;
+  const user_manager::UserID user_id_;
 
   DISALLOW_COPY_AND_ASSIGN(AppObserver);
 };
 
 MultiUserWindowManagerChromeOS::MultiUserWindowManagerChromeOS(
-    const std::string& current_user_id)
+    const user_manager::UserID& current_user_id)
     : current_user_id_(current_user_id),
+      null_user_id_(std::string(), std::string()),
       notification_blocker_(new MultiUserNotificationBlockerChromeOS(
           message_center::MessageCenter::Get(), current_user_id)),
       suppress_visibility_changes_(false),
@@ -275,7 +277,7 @@ void MultiUserWindowManagerChromeOS::Init() {
 
 void MultiUserWindowManagerChromeOS::SetWindowOwner(
     aura::Window* window,
-    const std::string& user_id) {
+    const user_manager::UserID& user_id) {
   // Make sure the window is valid and there was no owner yet.
   DCHECK(window);
   DCHECK(!user_id.empty());
@@ -307,17 +309,17 @@ void MultiUserWindowManagerChromeOS::SetWindowOwner(
     SetWindowVisibility(window, false, 0);
 }
 
-const std::string& MultiUserWindowManagerChromeOS::GetWindowOwner(
+const user_manager::UserID& MultiUserWindowManagerChromeOS::GetWindowOwner(
     aura::Window* window) const {
   WindowToEntryMap::const_iterator it = window_to_entry_.find(window);
   return it != window_to_entry_.end() ? it->second->owner()
-                                      : base::EmptyString();
+                                      : user_manager::EmptyUserID();
 }
 
 void MultiUserWindowManagerChromeOS::ShowWindowForUser(
     aura::Window* window,
-    const std::string& user_id) {
-  std::string previous_owner(GetUserPresentingWindow(window));
+    const user_manager::UserID& user_id) {
+  user_manager::UserID previous_owner(GetUserPresentingWindow(window));
   if (!ShowWindowForUserIntern(window, user_id))
     return;
   // The window switched to a new desktop and we have to switch to that desktop,
@@ -340,7 +342,7 @@ bool MultiUserWindowManagerChromeOS::AreWindowsSharedAmongUsers() const {
 }
 
 void MultiUserWindowManagerChromeOS::GetOwnersOfVisibleWindows(
-    std::set<std::string>* user_ids) const {
+    std::set<user_manager::UserID>* user_ids) const {
   for (WindowToEntryMap::const_iterator it = window_to_entry_.begin();
        it != window_to_entry_.end();
        ++it) {
@@ -351,25 +353,26 @@ void MultiUserWindowManagerChromeOS::GetOwnersOfVisibleWindows(
 
 bool MultiUserWindowManagerChromeOS::IsWindowOnDesktopOfUser(
     aura::Window* window,
-    const std::string& user_id) const {
-  const std::string& presenting_user = GetUserPresentingWindow(window);
+    const user_manager::UserID& user_id) const {
+  const user_manager::UserID& presenting_user = GetUserPresentingWindow(window);
   return presenting_user.empty() || presenting_user == user_id;
 }
 
-const std::string& MultiUserWindowManagerChromeOS::GetUserPresentingWindow(
+const user_manager::UserID& MultiUserWindowManagerChromeOS::GetUserPresentingWindow(
     aura::Window* window) const {
   WindowToEntryMap::const_iterator it = window_to_entry_.find(window);
   // If the window is not owned by anyone it is shown on all desktops and we
   // return the empty string.
   if (it == window_to_entry_.end())
-    return base::EmptyString();
+    return user_manager::EmptyUserID();
+
   // Otherwise we ask the object for its desktop.
   return it->second->show_for_user();
 }
 
 void MultiUserWindowManagerChromeOS::AddUser(content::BrowserContext* context) {
   Profile* profile = Profile::FromBrowserContext(context);
-  const std::string& user_id = multi_user_util::GetUserIDFromProfile(profile);
+  const user_manager::UserID& user_id = multi_user_util::GetUserIDFromProfile(profile);
   if (user_id_to_app_observer_.find(user_id) != user_id_to_app_observer_.end())
     return;
 
@@ -417,7 +420,7 @@ void MultiUserWindowManagerChromeOS::RemoveObserver(Observer* observer) {
 }
 
 void MultiUserWindowManagerChromeOS::ActiveUserChanged(
-    const std::string& user_id) {
+    const user_manager::UserID& user_id) {
   // This needs to be set before the animation starts.
   current_user_id_ = user_id;
 
@@ -534,17 +537,17 @@ bool MultiUserWindowManagerChromeOS::IsAnimationRunningForTest() {
   return animation_.get() != NULL && !animation_->IsAnimationFinished();
 }
 
-const std::string& MultiUserWindowManagerChromeOS::GetCurrentUserForTest()
+const user_manager::UserID& MultiUserWindowManagerChromeOS::GetCurrentUserForTest()
     const {
   return current_user_id_;
 }
 
 bool MultiUserWindowManagerChromeOS::ShowWindowForUserIntern(
     aura::Window* window,
-    const std::string& user_id) {
+    const user_manager::UserID& user_id) {
   // If there is either no owner, or the owner is the current user, no action
   // is required.
-  const std::string& owner = GetWindowOwner(window);
+  const user_manager::UserID& owner = GetWindowOwner(window);
   if (owner.empty() ||
       (owner == user_id && IsWindowOnDesktopOfUser(window, user_id)))
     return false;
@@ -598,7 +601,7 @@ void MultiUserWindowManagerChromeOS::SetWindowVisibility(
     if (window->parent() == system_modal_container) {
       // The window is system modal and we need to find the parent which owns
       // it so that we can switch to the desktop accordingly.
-      std::string user_id = GetUserPresentingWindow(window);
+      user_manager::UserID user_id = GetUserPresentingWindow(window);
       if (user_id.empty()) {
         aura::Window* owning_window = GetOwningWindowInTransientChain(window);
         DCHECK(owning_window);