Snap the snapped_dest_rect for backgrounds

For some reason the snapped_dest_rect for background image geometry
calculations was not actually snapped. Snapping it fixes cases where
we were not filling to the border without regressing any tests (in
fact improving some).

Probably fixes most of the background bugs resulting from the previous
big refactoring.

R=fmalita@chromium.org
BUG=912423,889426

Change-Id: Ieba65130e7f8c0e95db7375ab3a8188bfc78d8d5
Reviewed-on: https://chromium-review.googlesource.com/c/1372556
Commit-Queue: Stephen Chenney <schenney@chromium.org>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616014}
280 files changed