Refactors StackingController::GetDefaultParent to ash/wm/common

ash/wm/aura/wm_window_aura.cc

Index: ash/wm/aura/wm_window_aura.cc
diff --git a/ash/wm/aura/wm_window_aura.cc b/ash/wm/aura/wm_window_aura.cc
index 2a09ff622254e87dc08547ecd4e15e23d966b1a9..f483d93a51bbe53bdd1b26e286d2039c9ed4df18 100644
--- a/ash/wm/aura/wm_window_aura.cc
+++ b/ash/wm/aura/wm_window_aura.cc
@@ -190,6 +190,10 @@ bool WmWindowAura::IsVisible() const {
   return window_->IsVisible();
 }
 
+bool WmWindowAura::IsSystemModal() const {
+  return window_->GetProperty(aura::client::kModalKey) == ui::MODAL_TYPE_SYSTEM;
+}
+
 bool WmWindowAura::GetBoolProperty(WmWindowProperty key) {
   switch (key) {
     case WmWindowProperty::SNAP_CHILDREN_TO_PIXEL_BOUDARY: