This CL replaces user_manager::UserID with AccountId.

chrome/browser/ui/webui/chromeos/login/gaia_screen_handler.cc

Index: chrome/browser/ui/webui/chromeos/login/gaia_screen_handler.cc
diff --git a/chrome/browser/ui/webui/chromeos/login/gaia_screen_handler.cc b/chrome/browser/ui/webui/chromeos/login/gaia_screen_handler.cc
index 224dbdac9ef9a04b4d59322d9d61b11b92faa442..23db972ddbc9f5587f8cfbadadb1b558496ccb7b 100644
--- a/chrome/browser/ui/webui/chromeos/login/gaia_screen_handler.cc
+++ b/chrome/browser/ui/webui/chromeos/login/gaia_screen_handler.cc
@@ -428,27 +428,29 @@ void GaiaScreenHandler::HandleWebviewLoadAborted(
   UpdateState(error_reason);
 }
 
-std::string GaiaScreenHandler::GetCanonicalEmail(
+AccountId GaiaScreenHandler::GetAccountId(
     const std::string& authenticated_email,
     const std::string& gaia_id) const {
   const std::string sanitized_email = gaia::SanitizeEmail(authenticated_email);
 
   const std::string canonicalized_email =
       gaia::CanonicalizeEmail(sanitized_email);
+  const AccountId authenticated_id(
+      AccountId::FromUserEmailGaiaId(canonicalized_email, gaia_id));
+
   user_manager::UserManager* user_manager = user_manager::UserManager::Get();
-  if (user_manager && !user_manager->IsKnownUser(canonicalized_email)) {
-    std::string old_canonical_email;
-    if (user_manager->GetKnownUserCanonicalEmail(gaia_id,
-                                                 &old_canonical_email)) {
-      if (old_canonical_email != canonicalized_email) {
-        LOG(WARNING) << "Existing user '" << old_canonical_email
+  if (user_manager && !user_manager->IsKnownUser(authenticated_id)) {

[stevenjb, 2015/10/22 17:52:36]

nit: invert and early exit

[Alexander Alekseev, 2015/10/23 09:11:23]

Done.

+    AccountId old_account_id(EmptyAccountId());

[stevenjb, 2015/10/22 17:52:36]

It looks like this ship has sailed, but it is a bit jarring to me to discover
that EmptyAccountId is in the global namespace. (I spent some time looking for
the local definition).

[Alexander Alekseev, 2015/10/23 09:11:23]

The reason for this is to make EmptyAccountId() look like other reserved 
account ids (like GuestAccountId(), etc...). At least for now.

Probably it would be better to allow AccountId to be default-constructible, but
it is dangerous while we are in transition. Once the transition is finished,
probably it would become just AccountId() .

+    if (user_manager->GetKnownUserAccountId(authenticated_id,
+                                            &old_account_id)) {

[stevenjb, 2015/10/22 17:52:36]

nit: invert and early exit (same below, maybe combine)

[Alexander Alekseev, 2015/10/23 09:11:23]

Done.

+      if (old_account_id.GetUserEmail() != canonicalized_email) {
+        LOG(WARNING) << "Existing user '" << old_account_id.GetUserEmail()
                      << "' authenticated by alias '" << sanitized_email << "'.";
-        return old_canonical_email;
+        return old_account_id;
       }
     }
   }
-  // For compatibility reasons, sanitized email is used.
-  return sanitized_email;
+  return authenticated_id;
 }
 
 void GaiaScreenHandler::HandleCompleteAuthentication(
@@ -466,8 +468,8 @@ void GaiaScreenHandler::HandleCompleteAuthentication(
   const std::string sanitized_email = gaia::SanitizeEmail(email);
   Delegate()->SetDisplayEmail(sanitized_email);
 
-  const std::string canonical_email = GetCanonicalEmail(email, gaia_id);
-  UserContext user_context(canonical_email);
+  const AccountId account_id(GetAccountId(email, gaia_id));
+  UserContext user_context(account_id);

[achuithb, 2015/10/23 00:08:51]

Just inline invocation to GetAccountId(email, gaia_id) here

[Alexander Alekseev, 2015/10/23 09:11:23]

Done.

   user_context.SetGaiaID(gaia_id);
   user_context.SetKey(Key(password));
   user_context.SetAuthCode(auth_code);
@@ -500,7 +502,7 @@ void GaiaScreenHandler::HandleCompleteLogin(const std::string& gaia_id,
 
   // Consumer management enrollment is in progress.
   const std::string owner_email =
-      user_manager::UserManager::Get()->GetOwnerEmail();
+      user_manager::UserManager::Get()->GetOwnerId().GetUserEmail();
   if (typed_email != owner_email) {
     // Show Gaia sign-in screen again, since we only allow the owner to sign
     // in.
@@ -588,8 +590,8 @@ void GaiaScreenHandler::DoCompleteLogin(const std::string& gaia_id,
   DCHECK(!gaia_id.empty());
   const std::string sanitized_email = gaia::SanitizeEmail(typed_email);
   Delegate()->SetDisplayEmail(sanitized_email);
-  const std::string canonical_email = GetCanonicalEmail(typed_email, gaia_id);
-  UserContext user_context(canonical_email);
+  const AccountId account_id(GetAccountId(typed_email, gaia_id));
+  UserContext user_context(account_id);

[achuithb, 2015/10/23 00:08:51]

inline GetAccountId

[Alexander Alekseev, 2015/10/23 09:11:23]

Done.

   user_context.SetGaiaID(gaia_id);
   user_context.SetKey(Key(password));
   user_context.SetAuthFlow(using_saml
@@ -748,7 +750,7 @@ void GaiaScreenHandler::ShowGaiaScreenIfReady() {
     std::vector<std::string> input_methods =
         imm->GetInputMethodUtil()->GetHardwareLoginInputMethodIds();
     const std::string owner_im = SigninScreenHandler::GetUserLRUInputMethod(
-        user_manager::UserManager::Get()->GetOwnerEmail());
+        user_manager::UserManager::Get()->GetOwnerId().GetUserEmail());
     const std::string system_im = g_browser_process->local_state()->GetString(
         language_prefs::kPreferredKeyboardLayout);
 
@@ -836,7 +838,8 @@ void GaiaScreenHandler::LoadAuthExtension(bool force,
   context.is_enrolling_consumer_management = is_enrolling_consumer_management_;
 
   std::string gaia_id;
-  if (user_manager::UserManager::Get()->FindGaiaID(context.email, &gaia_id))
+  if (user_manager::UserManager::Get()->FindGaiaID(
+          AccountId::FromUserEmail(context.email), &gaia_id))
     context.gaia_id = gaia_id;
 
   if (Delegate()) {
@@ -846,7 +849,7 @@ void GaiaScreenHandler::LoadAuthExtension(bool force,
   if (!context.email.empty()) {
     context.gaps_cookie =
         user_manager::UserManager::Get()->GetKnownUserGAPSCookie(
-            gaia::CanonicalizeEmail(context.email));
+            AccountId::FromUserEmail(gaia::CanonicalizeEmail(context.email)));
   }
 
   populated_email_.clear();