AXAuraObjCache - clear focus client reference when window is destroyed

ui/views/accessibility/ax_aura_obj_cache.cc

Index: ui/views/accessibility/ax_aura_obj_cache.cc
diff --git a/ui/views/accessibility/ax_aura_obj_cache.cc b/ui/views/accessibility/ax_aura_obj_cache.cc
index 72abde71a028195511abdb3141eaa3d754a1cb58..9bd021e1d009dbe0b1f50cf3a8ac0ba272aa611f 100644
--- a/ui/views/accessibility/ax_aura_obj_cache.cc
+++ b/ui/views/accessibility/ax_aura_obj_cache.cc
@@ -35,6 +35,7 @@ AXAuraObjWrapper* AXAuraObjCache::GetOrCreate(aura::Window* window) {
     aura::Window* root_window = window->GetRootWindow();
     if (root_window) {
       focus_client_ = aura::client::GetFocusClient(root_window);
+      root_window->AddObserver(this);
       if (focus_client_)
         focus_client_->AddObserver(this);
     }
@@ -159,6 +160,10 @@ void AXAuraObjCache::OnWindowFocused(aura::Window* gained_focus,
     view->NotifyAccessibilityEvent(ui::AX_EVENT_FOCUS, true);
 }
 
+void AXAuraObjCache::OnWindowDestroying(aura::Window* window) {
+  focus_client_ = nullptr;
+}
+
 template <typename AuraViewWrapper, typename AuraView>
 AXAuraObjWrapper* AXAuraObjCache::CreateInternal(
     AuraView* aura_view,