This CL replaces std::string user_id in ash/* with AccountId.

chrome/browser/ui/ash/multi_user/multi_user_util.h

Index: chrome/browser/ui/ash/multi_user/multi_user_util.h
diff --git a/chrome/browser/ui/ash/multi_user/multi_user_util.h b/chrome/browser/ui/ash/multi_user/multi_user_util.h
index b85a4589a477eb943ac9a423effdc02250b5daaa..476e68fdffb1b2a6e38207f5b6030e9183209999 100644
--- a/chrome/browser/ui/ash/multi_user/multi_user_util.h
+++ b/chrome/browser/ui/ash/multi_user/multi_user_util.h
@@ -7,6 +7,7 @@
 
 #include <string>
 
+class AccountId;
 class Profile;
 
 namespace aura {
@@ -16,13 +17,13 @@ class Window;
 namespace multi_user_util {
 
 // Get the user id from a given profile.
-std::string GetUserIDFromProfile(Profile* profile);
+AccountId GetAccountIdFromProfile(Profile* profile);
 
 // Get the user id from an email address.
-std::string GetUserIDFromEmail(const std::string& email);
+AccountId GetAccountIdFromEmail(const std::string& email);
 
 // Get a profile for a given user id.
-Profile* GetProfileFromUserID(const std::string& user_id);
+Profile* GetProfileFromAccountId(const AccountId& account_id);
 
 // Get a profile for a |window|. Returns NULL if window belongs to no profile.
 Profile* GetProfileFromWindow(aura::Window* window);
@@ -32,9 +33,9 @@ Profile* GetProfileFromWindow(aura::Window* window);
 // return false.
 bool IsProfileFromActiveUser(Profile* profile);
 
-// Gets the current user. Note that this will only return for ChromeOS a string.
-// All other operating systems will get an empty string.
-const std::string& GetCurrentUserId();
+// Gets the current user. Note that this will return a valid AccountId for
+// ChromeOS only. All other operating systems will get an empty AccountId.
+const AccountId GetCurrentAccountId();
 
 // Move the window to the current user's desktop.
 void MoveWindowToCurrentDesktop(aura::Window* window);