Add kAccessibilityFocusFallsbackToWidget property

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 0ce0047c9987815064f5f795ccbb317198d53528..2c2d3d2baedc813876ebc729b739f6bad084eac6 100644
--- a/ui/views/accessibility/ax_aura_obj_cache.cc
+++ b/ui/views/accessibility/ax_aura_obj_cache.cc
@@ -160,8 +160,13 @@ View* AXAuraObjCache::GetFocusedView() {
   View* focused_view = focus_manager->GetFocusedView();
   if (focused_view)
     return focused_view;
+
+  // If no view is focused, falls back to accessibility focusable root view.
+  View* root_view = focused_widget->GetRootView();
+  if (root_view != nullptr && root_view->IsAccessibilityFocusable())
+    return root_view;
   else
-    return focused_widget->GetRootView();
+    return nullptr;
 }
 
 void AXAuraObjCache::OnWindowFocused(aura::Window* gained_focus,