Make ChildNodeRemovalNotifier::notify iterative

Now that we don't run script inside the removal process we don't need
the ChildNodesLazySnapshot or other machinery. That lets us rewrite the
entire removal process iteratively.

Review URL: https://codereview.chromium.org/289863007

git-svn-id: svn://svn.chromium.org/blink/trunk@174486 bbb929c8-8fbe-4397-9dbb-9b2b20218538