Change userid from a uint32_t to a string guid

mojo/shell/public/cpp/shell_client.h

Index: mojo/shell/public/cpp/shell_client.h
diff --git a/mojo/shell/public/cpp/shell_client.h b/mojo/shell/public/cpp/shell_client.h
index 36b93874ed492e54707ffcaf50174ef13ce7cc61..12c444a0e97208b283b2f0618ea73ef0fcf9fd87 100644
--- a/mojo/shell/public/cpp/shell_client.h
+++ b/mojo/shell/public/cpp/shell_client.h
@@ -33,8 +33,8 @@ class ShellClient {
   // Called exactly once before any other method.
   virtual void Initialize(Connector* connector,
                           const std::string& name,
-                          uint32_t id,
-                          uint32_t user_id = 0);
+                          const std::string& user_id,
+                          uint32_t id);
 
   // Called when a connection to this client is brokered by the shell. Override
   // to expose services to the remote application. Return true if the connection