Avoid copying Lengths (non-layout uses)

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: I8bcf5c6e6b99027110a20b1ef95d5f6607673ddc
Reviewed-on: https://chromium-review.googlesource.com/c/1411919
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623209}
7 files changed