[layoutng] Better handling of border and padding

This makes a few changes to the border/padding handling:
- Cache the computed value in a member variable instead of
  recomputing it for each child
- Makes sure to add block_end border and padding to the box size
- Takes border and padding into account for overflow size,
  and also for the constraint space size
- Remove functions that duplicate functionality from
  computeBorder/computePadding

BUG=635619

Review-Url: https://codereview.chromium.org/2330153002
Cr-Commit-Position: refs/heads/master@{#418368}
4 files changed