Move even more from WmShell to Shell

ash/wm/ash_focus_rules.cc

Index: ash/wm/ash_focus_rules.cc
diff --git a/ash/wm/ash_focus_rules.cc b/ash/wm/ash_focus_rules.cc
index 0dcc82df70a44a466988195c304e0719eace59e2..663358486dbadb5fe4e8d2a7e6eb2fa6ce8d86e4 100644
--- a/ash/wm/ash_focus_rules.cc
+++ b/ash/wm/ash_focus_rules.cc
@@ -80,7 +80,7 @@ aura::Window* AshFocusRules::GetNextActivatableWindow(
   // Start from the container of the most-recently-used window. If the list of
   // MRU windows is empty, then start from the container of the window that just
   // lost focus |ignore|.
-  MruWindowTracker* mru = WmShell::Get()->mru_window_tracker();
+  MruWindowTracker* mru = Shell::Get()->mru_window_tracker();
   std::vector<WmWindow*> windows = mru->BuildMruWindowList();
   aura::Window* starting_window =
       windows.empty() ? ignore : WmWindow::GetAuraWindow(windows[0]);