[LayoutNG] Avoid copying Lengths

Copying/assigning a Length is not just a trivial copy, so when possible,
use a const Length& to avoid making a copy (saving some small bit of
footprint and hopefully a few CPU cycles too).

Change-Id: Ib802f8e26c061e77bc8c0d8487a9dc8a8afe130b
Reviewed-on: https://chromium-review.googlesource.com/c/1411921
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622834}
3 files changed