ash: Explicitly teach various components about ShelfLayoutManager.

ash/system/tray/tray_background_view.cc

Index: ash/system/tray/tray_background_view.cc
diff --git a/ash/system/tray/tray_background_view.cc b/ash/system/tray/tray_background_view.cc
index 29d7fd0c53e074a990040dcb8c354257e7d567a5..f39cf32394942c8f33c2b63cd638bd408e4f26bd 100644
--- a/ash/system/tray/tray_background_view.cc
+++ b/ash/system/tray/tray_background_view.cc
@@ -390,7 +390,7 @@ void TrayBackgroundView::SetContentsBackground() {
 }
 
 ShelfLayoutManager* TrayBackgroundView::GetShelfLayoutManager() {
-  return ShelfLayoutManager::ForShelf(GetWidget()->GetNativeView());
+  return status_area_widget()->shelf_widget()->shelf_layout_manager();
 }
 
 void TrayBackgroundView::SetShelfAlignment(ShelfAlignment alignment) {