Change userid from a uint32_t to a string guid

mojo/shell/tests/shell/target.cc

Index: mojo/shell/tests/shell/target.cc
diff --git a/mojo/shell/tests/shell/target.cc b/mojo/shell/tests/shell/target.cc
index d0eeca254420a8c090a102a03d17e196c5608180..fe2c83cc0c49790c6d386f4a4b6a5641b356d5f1 100644
--- a/mojo/shell/tests/shell/target.cc
+++ b/mojo/shell/tests/shell/target.cc
@@ -24,7 +24,7 @@ class Target : public mojo::ShellClient {
  private:
   // mojo::ShellClient:
   void Initialize(mojo::Connector* connector, const std::string& name,
-                  uint32_t id, uint32_t user_id) override {
+                  const std::string& user_id, uint32_t id) override {
     CreateInstanceForFactoryTestPtr service;
     connector->ConnectToInterface("mojo:shell_unittest", &service);
     service->SetTargetID(id);