chromeos: Move SystemTrayDelegate::RequestShutdown to WmShell

ash/mus/bridge/wm_shell_mus.cc

Index: ash/mus/bridge/wm_shell_mus.cc
diff --git a/ash/mus/bridge/wm_shell_mus.cc b/ash/mus/bridge/wm_shell_mus.cc
index 42f3fb3ba2b57322205b35a9bc8ddfc7c12ee146..acfd3d1a3dd06df6eec5e5244c74bfabf261fa82 100644
--- a/ash/mus/bridge/wm_shell_mus.cc
+++ b/ash/mus/bridge/wm_shell_mus.cc
@@ -417,6 +417,10 @@ void WmShellMus::RemovePointerWatcher(views::PointerWatcher* watcher) {
   pointer_watcher_event_router_->RemovePointerWatcher(watcher);
 }
 
+void WmShellMus::RequestShutdown() {
+  NOTIMPLEMENTED();
+}
+
 bool WmShellMus::IsTouchDown() {
   // TODO: implement me, http://crbug.com/634967.
   // NOTIMPLEMENTED is too spammy here.