Move AccessibilityDelegate ownership to ash::WmShell

ash/aura/wm_shell_aura.h

Index: ash/aura/wm_shell_aura.h
diff --git a/ash/aura/wm_shell_aura.h b/ash/aura/wm_shell_aura.h
index ea87284805300c4c7df64244b38467a9e6aec272..5299f60e52114e0f658716d94b3541dab753600d 100644
--- a/ash/aura/wm_shell_aura.h
+++ b/ash/aura/wm_shell_aura.h
@@ -26,10 +26,8 @@ class ASH_EXPORT WmShellAura : public WmShell,
 
   static WmShellAura* Get();
 
-  // Called early in shutdown sequence.
-  void PrepareForShutdown();
-
   // WmShell:
+  void Shutdown() override;
   WmWindow* NewContainerWindow() override;
   WmWindow* GetFocusedWindow() override;
   WmWindow* GetActiveWindow() override;
@@ -55,7 +53,6 @@ class ASH_EXPORT WmShellAura : public WmShell,
   CreateScopedDisableInternalMouseAndKeyboard() override;
   void OnOverviewModeStarting() override;
   void OnOverviewModeEnded() override;
-  AccessibilityDelegate* GetAccessibilityDelegate() override;
   SessionStateDelegate* GetSessionStateDelegate() override;
   void AddActivationObserver(WmActivationObserver* observer) override;
   void RemoveActivationObserver(WmActivationObserver* observer) override;