Avoid Node references from AXObjectCache from leaking.

A real world example of this would be selecting an <option> item inside frame by keyboard. The node will not be deref()-ed until the topDocument() is detached. 

The issue was that AccessibilityMenuListOption is created in childrenChanged()
hook called when its RenderObject is being destroyed. This patch modifies AccessibilityMenuListPopup so it won't create AccessibilityMenuListOption if its
element is already detached.

BUG=270000

Review URL: https://chromiumcodereview.appspot.com/22990004

git-svn-id: svn://svn.chromium.org/blink/trunk@156197 bbb929c8-8fbe-4397-9dbb-9b2b20218538