[layoutng] Add a flag for content size vs border box size to ComputeMinMaxSize

And use it to simplify and speed up code in LayoutNGBlockFlow

Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: I56c486b66376497fc89b5c18ee5bbdf910573eb1
Reviewed-on: https://chromium-review.googlesource.com/1195502
Commit-Queue: Christian Biesinger <cbiesinger@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587154}