Change userid from a uint32_t to a string guid

mash/wm/window_manager_application.cc

Index: mash/wm/window_manager_application.cc
diff --git a/mash/wm/window_manager_application.cc b/mash/wm/window_manager_application.cc
index 8a60bb0ea698bb3b85d0a677abe3aed0baaaba58..6511c944617eb54bf92cf1901c77367d6b81a628 100644
--- a/mash/wm/window_manager_application.cc
+++ b/mash/wm/window_manager_application.cc
@@ -105,8 +105,8 @@ void WindowManagerApplication::OnAcceleratorRegistrarDestroyed(
 
 void WindowManagerApplication::Initialize(mojo::Connector* connector,
                                           const std::string& url,
-                                          uint32_t id,
-                                          uint32_t user_id) {
+                                          const std::string& user_id,
+                                          uint32_t id) {
   connector_ = connector;
   tracing_.Initialize(connector, url);