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

ash/shell/shell_delegate_impl.cc

Index: ash/shell/shell_delegate_impl.cc
diff --git a/ash/shell/shell_delegate_impl.cc b/ash/shell/shell_delegate_impl.cc
index e87aab0b52bb8acf4cd1862c5dc6dd5ff92c43be..5677b9b7570d798acca75adf1b64f1eab6c90237 100644
--- a/ash/shell/shell_delegate_impl.cc
+++ b/ash/shell/shell_delegate_impl.cc
@@ -117,7 +117,7 @@ class SessionStateDelegateImpl : public SessionStateDelegate {
   gfx::ImageSkia GetAvatarImageForWindow(aura::Window* window) const override {
     return gfx::ImageSkia();
   }
-  void SwitchActiveUser(const std::string& user_id) override {}
+  void SwitchActiveUser(const AccountId& account_id) override {}
   void CycleActiveUser(CycleUser cycle_user) override {}
   bool IsMultiProfileAllowedByPrimaryUserPolicy() const override {
     return true;