Introduce window container to be used by lock screen app windows

ash/wm/lock_layout_manager.h

Index: ash/wm/lock_layout_manager.h
diff --git a/ash/wm/lock_layout_manager.h b/ash/wm/lock_layout_manager.h
index 2674e2ba3fb0fca31656dade7c4ab9a34bdfacd0..b877fc07fe4fccf33d8d8a5580bb2da8ab98a9b5 100644
--- a/ash/wm/lock_layout_manager.h
+++ b/ash/wm/lock_layout_manager.h
@@ -66,11 +66,15 @@ class ASH_EXPORT LockLayoutManager
   void OnKeyboardBoundsChanging(const gfx::Rect& new_bounds) override;
   void OnKeyboardClosed() override;
 
- private:
+ protected:
   // Adjusts the bounds of all managed windows when the display area changes.
   // This happens when the display size, work area insets has changed.
   void AdjustWindowsForWorkAreaChange(const wm::WMEvent* event);
 
+  aura::Window* window() { return window_; }
+  aura::Window* root_window() { return root_window_; }
+
+ private:
   aura::Window* window_;
   aura::Window* root_window_;