Converts remaining usage of WmLayoutManager to aura::LayoutManager

ash/wm/window_positioning_utils.cc

Index: ash/wm/window_positioning_utils.cc
diff --git a/ash/wm/window_positioning_utils.cc b/ash/wm/window_positioning_utils.cc
index 03df182a8af65c486d0057f37569324b31dc8a6c..f733954912234bafea97c2b8cfdc140557dec81c 100644
--- a/ash/wm/window_positioning_utils.cc
+++ b/ash/wm/window_positioning_utils.cc
@@ -152,7 +152,8 @@ void SetBoundsInScreen(WmWindow* window,
       // All containers that uses screen coordinates must have valid window ids.
       DCHECK_GE(container_id, 0);
       // Don't move modal background.
-      if (!SystemModalContainerLayoutManager::IsModalBackground(window))
+      if (!SystemModalContainerLayoutManager::IsModalBackground(
+              window->aura_window()))
         dst_container = dst_root->GetChildByShellWindowId(container_id);
     }