Use LayoutUnit for all border-*-width logic

This patch uses LayoutUnit for the logic of converting back and
forth between the stored int and the returned float value
instead of having this logic in main ComputedStyle.

Diff: https://gist.github.com/nainar/5d4e8db04884cb0dccb4874a0b23ef0d/revisions

BUG=628043

Review-Url: https://codereview.chromium.org/2904453002
Cr-Commit-Position: refs/heads/master@{#474181}
3 files changed