Move AccessibilityDelegate ownership to ash::WmShell

ash/aura/wm_shell_aura.cc

Index: ash/aura/wm_shell_aura.cc
diff --git a/ash/aura/wm_shell_aura.cc b/ash/aura/wm_shell_aura.cc
index c5a4db581aa332738d9ff87ba3c1e542f71c9f24..5ac8a64f76a0a59ba18ad9af27b0bb26ee19062c 100644
--- a/ash/aura/wm_shell_aura.cc
+++ b/ash/aura/wm_shell_aura.cc
@@ -49,12 +49,14 @@ WmShellAura::~WmShellAura() {
   WmShell::Set(nullptr);
 }
 
-void WmShellAura::PrepareForShutdown() {
+void WmShellAura::Shutdown() {
   if (added_activation_observer_)
     Shell::GetInstance()->activation_client()->RemoveObserver(this);
 
   if (added_display_observer_)
     Shell::GetInstance()->window_tree_host_manager()->RemoveObserver(this);
+
+  WmShell::Shutdown();
 }
 
 WmWindow* WmShellAura::NewContainerWindow() {
@@ -163,10 +165,6 @@ void WmShellAura::OnOverviewModeEnded() {
   FOR_EACH_OBSERVER(ShellObserver, *shell_observers(), OnOverviewModeEnded());
 }
 
-AccessibilityDelegate* WmShellAura::GetAccessibilityDelegate() {
-  return Shell::GetInstance()->accessibility_delegate();
-}
-
 SessionStateDelegate* WmShellAura::GetSessionStateDelegate() {
   return Shell::GetInstance()->session_state_delegate();
 }