AXMenuListPopup and RenderMenuList should not use confusingAndOftenMisusedAttached()

Instead of checking for attached() in this code we should just defer the
creation of the AXObjects for the children until updating the selected
option.

BUG=316466

Review URL: https://codereview.chromium.org/70623005

git-svn-id: svn://svn.chromium.org/blink/trunk@161948 bbb929c8-8fbe-4397-9dbb-9b2b20218538