Avoid re-computing rel-pos offset during paint.

This is an expensive operation. See LayoutBoxModelObject::RelativePositionOffset.

Bug: 984965

Change-Id: I487d2eaa96ee63c8cff88d12a062feec53ddb05a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1726896
Reviewed-by: Mason Freed <masonfreed@chromium.org>
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682944}