Change userid from a uint32_t to a string guid

mojo/shell/public/cpp/connection.h

Index: mojo/shell/public/cpp/connection.h
diff --git a/mojo/shell/public/cpp/connection.h b/mojo/shell/public/cpp/connection.h
index 1825d9b4ebceffb43b00ca2a044b1d8b267580b3..0654423d62e38bfdae9a6a914357b3715e334d07 100644
--- a/mojo/shell/public/cpp/connection.h
+++ b/mojo/shell/public/cpp/connection.h
@@ -81,7 +81,7 @@ class Connection {
   // callback being fired, this will return the value passed via Connect().
   // After the Connect() callback (call AddRemoteIDCallback to register one)
   // this will return the actual user id the shell ran the target as.
-  virtual uint32_t GetRemoteUserID() const = 0;
+  virtual const std::string& GetRemoteUserID() const = 0;
 
   // Register a handler to receive an error notification on the pipe to the
   // remote application's InterfaceProvider.