window-service: use ceiling when converting to pixels

ClientRoot was using floor, which doesn't match that used by the client
(ceiling). This converts the two to use the same function (ceiling).

BUG=952095
TEST=covered by test

Change-Id: Ia0f78c63ace3987f2182adaf9783bd69ccc9171c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1566157
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Jun Mukai <mukai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#650417}