Change userid from a uint32_t to a string guid

mojo/shell/public/cpp/lib/shell_client.cc

Index: mojo/shell/public/cpp/lib/shell_client.cc
diff --git a/mojo/shell/public/cpp/lib/shell_client.cc b/mojo/shell/public/cpp/lib/shell_client.cc
index a06c875507e101c6d83b91c4f753c61523fab3ca..f5441ce12e11a0dde156c74fa2c7d030867f033f 100644
--- a/mojo/shell/public/cpp/lib/shell_client.cc
+++ b/mojo/shell/public/cpp/lib/shell_client.cc
@@ -10,7 +10,7 @@ ShellClient::ShellClient() {}
 ShellClient::~ShellClient() {}
 
 void ShellClient::Initialize(Connector* connector, const std::string& name,
-                             uint32_t id, uint32_t user_id) {
+                             const std::string& user_id, uint32_t id) {
 }
 
 bool ShellClient::AcceptConnection(Connection* connection) {