Folds methods in WmShellCommon to WmShell

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 929c3ace52fecfc1ba54a834ed27a4f4f4ca2fa2..e6e1c63f4c68545623cda4d1952d08a0d766b42c 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|.
-  ash::MruWindowTracker* mru = ash::Shell::GetInstance()->mru_window_tracker();
+  MruWindowTracker* mru = WmShell::Get()->mru_window_tracker();
   std::vector<WmWindow*> windows = mru->BuildMruWindowList();
   aura::Window* starting_window =
       windows.empty() ? ignore : WmWindowAura::GetAuraWindow(windows[0]);