Make ComboboxListener work around being deleted from OnSelectedIndexChanged().

ui/views/controls/combobox/combobox.cc

Index: ui/views/controls/combobox/combobox.cc
diff --git a/ui/views/controls/combobox/combobox.cc b/ui/views/controls/combobox/combobox.cc
index d726fe2032f5528c0a56925f9378ff27d2de58a9..264e3560b80cd986f3db96f125fe98e1b718c59e 100644
--- a/ui/views/controls/combobox/combobox.cc
+++ b/ui/views/controls/combobox/combobox.cc
@@ -468,10 +468,11 @@ void Combobox::ShowDropDownMenu(ui::MenuSourceType source_type) {
 }
 
 void Combobox::OnSelectionChanged() {
-  if (listener_)
-    listener_->OnSelectedIndexChanged(this);
   NotifyAccessibilityEvent(ui::AccessibilityTypes::EVENT_VALUE_CHANGED, false);
   SchedulePaint();
+  if (listener_)
+    listener_->OnSelectedIndexChanged(this);
+  // |this| may now be deleted.
 }
 
 int Combobox::MenuCommandToIndex(int menu_command_id) const {