[LayoutNG] Refactor huge flex Layout method

Also add and clarify some comments.

No behavior change intended.

Bug: 845235
Change-Id: Ic1b816b2b944c76014ba74175a693c5903960333
Reviewed-on: https://chromium-review.googlesource.com/c/1401116
Commit-Queue: David Grogan <dgrogan@chromium.org>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622925}
4 files changed