ash: Move ShelfWidget ownership to WmShelf and refactor access to it

ash/aura/wm_shelf_aura.cc

Index: ash/aura/wm_shelf_aura.cc
diff --git a/ash/aura/wm_shelf_aura.cc b/ash/aura/wm_shelf_aura.cc
index 33c5d02d87f7f9e76ab70ef334e86d76508bfab9..36e428e081507b0f7053c43d5bac6c1bcc495646 100644
--- a/ash/aura/wm_shelf_aura.cc
+++ b/ash/aura/wm_shelf_aura.cc
@@ -51,8 +51,8 @@ WmDimmerView* WmShelfAura::CreateDimmerView(bool disable_animations_for_test) {
   return DimmerView::Create(this, disable_animations_for_test);
 }
 
-void WmShelfAura::SetShelfLayoutManager(ShelfLayoutManager* manager) {
-  WmShelf::SetShelfLayoutManager(manager);
+void WmShelfAura::CreateShelfWidget(WmWindow* root) {
+  WmShelf::CreateShelfWidget(root);
   bezel_event_handler_.reset(new ShelfBezelEventHandler(this));
 }