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

chromeos/login/auth/user_context.cc

Index: chromeos/login/auth/user_context.cc
diff --git a/chromeos/login/auth/user_context.cc b/chromeos/login/auth/user_context.cc
index e5b3fe0951949df8a3a14685971a20c757e66721..08977f5e1fe74fd2d20cd2bcf6b8991e3a3c2c67 100644
--- a/chromeos/login/auth/user_context.cc
+++ b/chromeos/login/auth/user_context.cc
@@ -8,14 +8,14 @@
 namespace chromeos {
 
 UserContext::UserContext()
-    : is_using_oauth_(true),
+    : user_id_(std::string() /* gaia_id */, std::string() /* user_email */),
+      is_using_oauth_(true),
       auth_flow_(AUTH_FLOW_OFFLINE),
       user_type_(user_manager::USER_TYPE_REGULAR) {
 }
 
 UserContext::UserContext(const UserContext& other)
     : user_id_(other.user_id_),
-      gaia_id_(other.gaia_id_),
       key_(other.key_),
       auth_code_(other.auth_code_),
       refresh_token_(other.refresh_token_),
@@ -29,20 +29,22 @@ UserContext::UserContext(const UserContext& other)
       device_id_(other.device_id_) {
 }
 
-UserContext::UserContext(const std::string& user_id)
-    : user_id_(login::CanonicalizeUserID(user_id)),
+UserContext::UserContext(const user_manager::UserID& user_id)
+    : user_id_(user_id),
       is_using_oauth_(true),
       auth_flow_(AUTH_FLOW_OFFLINE),
       user_type_(user_manager::USER_TYPE_REGULAR) {
+  user_id_.SetUserEmail(login::CanonicalizeUserID(user_id.GetUserEmail()));
 }
 
 UserContext::UserContext(user_manager::UserType user_type,
-                         const std::string& user_id)
-    : is_using_oauth_(true),
+                         const user_manager::UserID& user_id)
+    : user_id_(user_id),
+      is_using_oauth_(true),
       auth_flow_(AUTH_FLOW_OFFLINE),
       user_type_(user_type) {
   if (user_type_ == user_manager::USER_TYPE_REGULAR)
-    user_id_ = login::CanonicalizeUserID(user_id);
+    user_id_.SetUserEmail(login::CanonicalizeUserID(user_id.GetUserEmail()));
   else
     user_id_ = user_id;
 }
@@ -51,7 +53,7 @@ UserContext::~UserContext() {
 }
 
 bool UserContext::operator==(const UserContext& context) const {
-  return context.user_id_ == user_id_ && context.gaia_id_ == gaia_id_ &&
+  return context.user_id_ == user_id_ &&
          context.key_ == key_ && context.auth_code_ == auth_code_ &&
          context.refresh_token_ == refresh_token_ &&
          context.access_token_ == access_token_ &&
@@ -66,14 +68,10 @@ bool UserContext::operator!=(const UserContext& context) const {
   return !(*this == context);
 }
 
-const std::string& UserContext::GetUserID() const {
+const user_manager::UserID& UserContext::GetUserID() const {
   return user_id_;
 }
 
-const std::string& UserContext::GetGaiaID() const {
-  return gaia_id_;
-}
-
 const Key* UserContext::GetKey() const {
   return &key_;
 }
@@ -127,12 +125,8 @@ bool UserContext::HasCredentials() const {
          !auth_code_.empty();
 }
 
-void UserContext::SetUserID(const std::string& user_id) {
-  user_id_ = login::CanonicalizeUserID(user_id);
-}
-
-void UserContext::SetGaiaID(const std::string& gaia_id) {
-  gaia_id_ = gaia_id;
+void UserContext::SetUserID(const user_manager::UserID& user_id) {
+  user_id_.SetUserEmail(login::CanonicalizeUserID(user_id.GetUserEmail()));
 }
 
 void UserContext::SetKey(const Key& key) {