Make NodeIterator.detach() a no-op

Make NodeIterator.detach() a no-op as per the DOM specification:
http://dom.spec.whatwg.org/#dom-nodeiterator-detach

This makes the code simpler, and a bit more efficient.

Firefox already does this since version 22:
https://bugzilla.mozilla.org/show_bug.cgi?id=823549

However, this method is not a no-op in IE11.

R=arv@chromium.org, tkent@chromium.org
BUG=367558
TEST=fast/dom/NodeIterator/detach-no-op.html

Review URL: https://codereview.chromium.org/256013002

git-svn-id: svn://svn.chromium.org/blink/trunk@172768 bbb929c8-8fbe-4397-9dbb-9b2b20218538