Change userid from a uint32_t to a string guid

mash/browser_driver/browser_driver_application_delegate.h

Index: mash/browser_driver/browser_driver_application_delegate.h
diff --git a/mash/browser_driver/browser_driver_application_delegate.h b/mash/browser_driver/browser_driver_application_delegate.h
index cbe63cf97656a0b1a51adfe72fe34c5ca9a24213..03d67d38949c5b16c763795fc4cbfa9c4b080388 100644
--- a/mash/browser_driver/browser_driver_application_delegate.h
+++ b/mash/browser_driver/browser_driver_application_delegate.h
@@ -28,7 +28,7 @@ class BrowserDriverApplicationDelegate : 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;
 
   // mus::mojom::AcceleratorHandler: