AXAuraObjCache - clear focus client reference when window is destroyed

BUG=591752

Review URL: https://codereview.chromium.org/1765583002

Cr-Commit-Position: refs/heads/master@{#379087}
(cherry picked from commit 3913fbd)

Review URL: https://codereview.chromium.org/1767383002 .

Cr-Commit-Position: refs/branch-heads/2661@{crosswalk-project#121}
Cr-Branched-From: ef6f6ae-refs/heads/master@{#378081}

Author: Evan Stade

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,

ui/views/accessibility/ax_aura_obj_cache.h

@@ -12,6 +12,7 @@
 
 #include "base/macros.h"
 #include "ui/aura/client/focus_change_observer.h"
+#include "ui/aura/window_observer.h"
 #include "ui/views/views_export.h"
 
 namespace base {
@@ -32,7 +33,8 @@ class Widget;
 
 // A cache responsible for assigning id's to a set of interesting Aura views.
 class VIEWS_EXPORT AXAuraObjCache
-    : public aura::client::FocusChangeObserver {
+    : public aura::client::FocusChangeObserver,
+      public aura::WindowObserver {
  public:
   // Get the single instance of this class.
   static AXAuraObjCache* GetInstance();
@@ -86,6 +88,9 @@ class VIEWS_EXPORT AXAuraObjCache
   void OnWindowFocused(aura::Window* gained_focus,
                        aura::Window* lost_focus) override;
 
+  // aura::WindowObserver override.
+  void OnWindowDestroying(aura::Window* window) override;
+
   template <typename AuraViewWrapper, typename AuraView>
   AXAuraObjWrapper* CreateInternal(
       AuraView* aura_view,