Change userid from a uint32_t to a string guid

ash/mus/sysui_application.h

Index: ash/mus/sysui_application.h
diff --git a/ash/mus/sysui_application.h b/ash/mus/sysui_application.h
index c597651e80a6415c4d526e50d3e24563af424f1d..78d159904384c4a285f23ecdf3547189ac9aac47 100644
--- a/ash/mus/sysui_application.h
+++ b/ash/mus/sysui_application.h
@@ -24,8 +24,8 @@ class SysUIApplication : public mojo::ShellClient {
   // mojo::ShellClient:
   void Initialize(mojo::Connector* connector,
                   const std::string& url,
-                  uint32_t id,
-                  uint32_t user_id) override;
+                  const std::string& user_id,
+                  uint32_t id) override;
   bool AcceptConnection(mojo::Connection* connection) override;
 
   mojo::TracingImpl tracing_;