Change userid from a uint32_t to a string guid

mash/screenlock/screenlock.cc

Index: mash/screenlock/screenlock.cc
diff --git a/mash/screenlock/screenlock.cc b/mash/screenlock/screenlock.cc
index e3bd22924caf9793186781a1b5c703e8a68c5f5a..42f2b0453f3f9871170137726b585b2c33925734 100644
--- a/mash/screenlock/screenlock.cc
+++ b/mash/screenlock/screenlock.cc
@@ -76,7 +76,7 @@ Screenlock::Screenlock() {}
 Screenlock::~Screenlock() {}
 
 void Screenlock::Initialize(mojo::Connector* connector, const std::string& url,
-                            uint32_t id, uint32_t user_id) {
+                            const std::string& user_id, uint32_t id) {
   tracing_.Initialize(connector, url);
 
   mash::shell::mojom::ShellPtr mash_shell;