Implement NGBoxStrut::ConvertToPhysical with tests

Bug: 740993
Change-Id: I742b2aa94048e55204d204e0ce6fcaee7b0af6d8
Reviewed-on: https://chromium-review.googlesource.com/567776
Commit-Queue: Aleks Totic <atotic@chromium.org>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486196}
3 files changed