Move AccessibilityDelegate ownership to ash::WmShell

ash/common/wm_shell.cc

Index: ash/common/wm_shell.cc
diff --git a/ash/common/wm_shell.cc b/ash/common/wm_shell.cc
index bb012d7abb237310a4c141fbfe0bbdaf163b45e2..15bebd1f69a180003d6c11e0f146dde8bcfe46fc 100644
--- a/ash/common/wm_shell.cc
+++ b/ash/common/wm_shell.cc
@@ -6,6 +6,7 @@
 
 #include <utility>
 
+#include "ash/common/accessibility_delegate.h"
 #include "ash/common/focus_cycler.h"
 #include "ash/common/keyboard/keyboard_ui.h"
 #include "ash/common/shell_delegate.h"
@@ -45,9 +46,15 @@ WmShell* WmShell::Get() {
 void WmShell::Initialize() {
   // Some delegates access WmShell during their construction. Create them here
   // instead of the WmShell constructor.
+  accessibility_delegate_.reset(delegate_->CreateAccessibilityDelegate());
   media_delegate_.reset(delegate_->CreateMediaDelegate());
 }
 
+void WmShell::Shutdown() {
+  media_delegate_.reset();

[James Cook, 2016/07/14 23:31:46]

Not strictly necessary, but a little clearer I think.

[msw, 2016/07/15 15:33:14]

I'd be wary of adding explicit destruction if none is needed. Shell has plenty
of relative lifetime (ctor/dtor ordering) issues, and this may imply that these
members need to be destroyed before the dtor or in some specific order. WDYT?

Maybe copy the comment from shell.cc if it applies to |accessibility_delegate_|:
  // Depends on |focus_client_|, so must be destroyed before.

[James Cook, 2016/07/15 17:03:06]

Done. Removed media_delegate_.reset(). Added a comment that
accessibility_delegate_ accesses WmShell in its destructor, which is the actual
problem.

+  accessibility_delegate_.reset();
+}
+
 void WmShell::OnMaximizeModeStarted() {
   FOR_EACH_OBSERVER(ShellObserver, shell_observers_, OnMaximizeModeStarted());
 }