[LayoutNG] Fix flexbox height calculation

We were double counting the block-start border.

Bug: 845235
Change-Id: I7a1217c415c800fb5d5517155b63da84e720f8b7
Reviewed-on: https://chromium-review.googlesource.com/c/1396316
Commit-Queue: David Grogan <dgrogan@chromium.org>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#620195}
2 files changed