Always assert needsAttach() in ~ContainerNode

This should be true even if we're not in Oilpan mode. By the time the
node is being destroyed we better be detached already. Indeed the
call to removeDetachedChildren() has this ASSERT too.

Review URL: https://codereview.chromium.org/287193016

git-svn-id: svn://svn.chromium.org/blink/trunk@174721 bbb929c8-8fbe-4397-9dbb-9b2b20218538