Reland r256396 Add views implementation for AXTreeSource.

ui/views/widget/widget.cc

Index: ui/views/widget/widget.cc
diff --git a/ui/views/widget/widget.cc b/ui/views/widget/widget.cc
index 68c339ab9509a76ffe0781efddfed1f461f8a699..6be502d8b1e8fbb8dc9f492782f28193de8c7688 100644
--- a/ui/views/widget/widget.cc
+++ b/ui/views/widget/widget.cc
@@ -427,6 +427,13 @@ void Widget::ViewHierarchyChanged(
       focus_manager->ViewRemoved(details.child);
     ViewStorage::GetInstance()->ViewRemoved(details.child);
     native_widget_->ViewRemoved(details.child);
+
+    // Many observers do not remove themselves during destruction, so we must
+    // not notify once we destroy our |root_view_|.
+    if (root_view_.get()) {
+      FOR_EACH_OBSERVER(
+          WidgetObserver, observers_, OnViewRemoved(this, details.child));
+    }
   }
 }