[LayoutNG] Refactor main-loop of block layout algorithm.

This just simplifies things by having clear paths for each type of child.
There should be no functional changes.

BUG=635619
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_layout_ng

Review-Url: https://codereview.chromium.org/2883353005
Cr-Commit-Position: refs/heads/master@{#473077}
3 files changed