Change userid from a uint32_t to a string guid

mash/login/login.h

Index: mash/login/login.h
diff --git a/mash/login/login.h b/mash/login/login.h
index 2a70b1ab5ebf319fadc49d803b6bc74a5c549077..53fcbf656e999d387d0df9a71b62bc0ca7ba0b36 100644
--- a/mash/login/login.h
+++ b/mash/login/login.h
@@ -30,7 +30,7 @@ class Login : public mojo::ShellClient {
  private:
   // 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_;