mash: Migrate ScopedTargetRootWindow to //ash/common

ash/common/wm/mru_window_tracker.cc

Index: ash/common/wm/mru_window_tracker.cc
diff --git a/ash/common/wm/mru_window_tracker.cc b/ash/common/wm/mru_window_tracker.cc
index bea899b05510e9ba7afc26c051cc22143a55220c..0857d995defccc06d6fbf0bc98d53a610130e061 100644
--- a/ash/common/wm/mru_window_tracker.cc
+++ b/ash/common/wm/mru_window_tracker.cc
@@ -50,7 +50,7 @@ MruWindowTracker::WindowList BuildWindowListInternal(
     const std::list<WmWindow*>* mru_windows,
     const CanActivateWindowPredicate& should_include_window_predicate) {
   MruWindowTracker::WindowList windows;
-  WmWindow* active_root = WmShell::Get()->GetRootWindowForNewWindows();
+  WmWindow* active_root = WmShell::Get()->root_window_for_new_windows();
   for (WmWindow* window : WmShell::Get()->GetAllRootWindows()) {
     if (window == active_root)
       continue;