[LayoutNG] Move decision if a inline node is empty to CollectInlines.

This is to simplify bfc resolving "abort" code. Inside NGLineBreaker we
should be able to always have a resolved BFC offset with this change.

In a followup i'll move the unpositioned floats behaviour to a separate
walk if needed.

BUG=635619

Review-Url: https://codereview.chromium.org/2970343002
Cr-Commit-Position: refs/heads/master@{#485655}
9 files changed