Change userid from a uint32_t to a string guid

mojo/shell/shell.h

Index: mojo/shell/shell.h
diff --git a/mojo/shell/shell.h b/mojo/shell/shell.h
index 776719874dcbdf0ae2c91d44d3a096f2e8f9fb0b..4a5dcb2508b66c2cda7c869fb626b597414e8124 100644
--- a/mojo/shell/shell.h
+++ b/mojo/shell/shell.h
@@ -122,11 +122,11 @@ class Shell : public ShellClient {
                            mojom::ShellClientRequest* request);
 
   // Called from the instance implementing mojom::Shell. |user_id| must be
-  // resolved by the instance (i.e. must not be mojom::Connector::kUserInherit).
+  // resolved by the instance (i.e. must not be mojom::kInheritUserID).
   void CreateInstanceForFactory(
       mojom::ShellClientFactoryPtr factory,
-      const String& name,
-      uint32_t user_id,
+      const std::string& name,
+      const std::string& user_id,
       mojom::CapabilityFilterPtr filter,
       mojom::PIDReceiverRequest pid_receiver);
   // Called from the instance implementing mojom::Shell.