Fix detached Attr nodes interaction with NodeIterator

- Don't register NodeIterator to document when attaching to Attr node.
-- NodeIterator is registered to its document to receive updateForNodeRemoval notifications.
-- However it wouldn't make sense on Attr nodes, as they never have children.

BUG=572537

Review URL: https://codereview.chromium.org/1577213003

Cr-Commit-Position: refs/heads/master@{#369687}