Change userid from a uint32_t to a string guid

mash/task_viewer/task_viewer.cc

Index: mash/task_viewer/task_viewer.cc
diff --git a/mash/task_viewer/task_viewer.cc b/mash/task_viewer/task_viewer.cc
index 8d183e262cc29345c93f52f3465b3b7c4bf044fe..e3af3da154d9962545a152f0b309df08540107d8 100644
--- a/mash/task_viewer/task_viewer.cc
+++ b/mash/task_viewer/task_viewer.cc
@@ -260,7 +260,8 @@ TaskViewer::~TaskViewer() {}
 
 void TaskViewer::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);
 
   aura_init_.reset(new views::AuraInit(connector, "views_mus_resources.pak"));