[layoutng] Move LayoutBox knowledge out of the NGBoxIterator

There is no need for the iterator to know about layout
objects/boxes; we can just add and use a nextSibling() method
on NGBox. This will simplify a possible future change to allow
NGBox objects to work even without an attached layout object.
(Such functionality would be especially useful for unit tests
at the moment)

R=eae@chromium.org,ikilpatrick@chromium.org,glebl@chromium.org
BUG=635619

Review-Url: https://codereview.chromium.org/2232423002
Cr-Commit-Position: refs/heads/master@{#411497}
5 files changed