Avoid copying Lengths (in layout sans NG)

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: Idd1633b4198a8af62c46c27c36751e7961eb356f
Reviewed-on: https://chromium-review.googlesource.com/c/1411693
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#622959}
24 files changed