chromeos: Eliminate WmWindow from //ash/system

ash/shelf/shelf_widget.cc

Index: ash/shelf/shelf_widget.cc
diff --git a/ash/shelf/shelf_widget.cc b/ash/shelf/shelf_widget.cc
index 96fdc65fdc074fef22018daba05e0721ef7c67c6..76978e09b937f0ac6330a25188b32749245d60b1 100644
--- a/ash/shelf/shelf_widget.cc
+++ b/ash/shelf/shelf_widget.cc
@@ -182,7 +182,8 @@ ShelfWidget::~ShelfWidget() {
 void ShelfWidget::CreateStatusAreaWidget(WmWindow* status_container) {
   DCHECK(status_container);
   DCHECK(!status_area_widget_);
-  status_area_widget_ = new StatusAreaWidget(status_container, wm_shelf_);
+  status_area_widget_ =
+      new StatusAreaWidget(status_container->aura_window(), wm_shelf_);
   status_area_widget_->CreateTrayViews();
   // NOTE: Container may be hidden depending on login/display state.
   status_area_widget_->Show();