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

ash/mus/bridge/wm_shelf_mus.cc

Index: ash/mus/bridge/wm_shelf_mus.cc
diff --git a/ash/mus/bridge/wm_shelf_mus.cc b/ash/mus/bridge/wm_shelf_mus.cc
index 0b03fcff97c8928fc6b9974b89897563e042dc48..f2f08421abef95daa248b9492ca70618522468e9 100644
--- a/ash/mus/bridge/wm_shelf_mus.cc
+++ b/ash/mus/bridge/wm_shelf_mus.cc
@@ -5,7 +5,6 @@
 #include "ash/mus/bridge/wm_shelf_mus.h"
 
 #include "ash/common/shelf/shelf_widget.h"
-#include "ash/common/shell_window_ids.h"
 #include "ash/common/wm_root_window_controller.h"
 #include "ash/common/wm_shell.h"
 #include "ash/common/wm_window.h"
@@ -17,21 +16,19 @@ WmShelfMus::WmShelfMus(WmRootWindowController* root_window_controller) {
   DCHECK(root_window_controller);
   WmShell::Get()->CreateShelfDelegate();
   WmWindow* root = root_window_controller->GetWindow();
-  shelf_widget_.reset(new ShelfWidget(
-      root->GetChildByShellWindowId(kShellWindowId_ShelfContainer),
-      root->GetChildByShellWindowId(kShellWindowId_StatusContainer), this));
+  CreateShelfWidget(root);
   InitializeShelf();
   WmShell::Get()->NotifyShelfCreatedForRootWindow(root);
-  shelf_widget_->PostCreateShelf();
+  shelf_widget()->PostCreateShelf();
 }
 
 WmShelfMus::~WmShelfMus() {
-  shelf_widget_.reset();
+  DestroyShelfWidget();
   ShutdownShelf();
 }
 
 void WmShelfMus::WillDeleteShelfLayoutManager() {
-  shelf_widget_->Shutdown();
+  ShutdownShelfWidget();
   WmShelf::WillDeleteShelfLayoutManager();
 }