Currently BackgroundImageGeometry computes a tiling geometry in the
presence of borders.  This is correct, because (at least for
background-origin: padding-box/content-box) backgrounds are repeated
logically underneath the border.

But if the border is opaque and fully obscures the background, these
extra tiles are redundant.  What's more, tiling forces us to use
image shaders for painting, unnecessarily.

This CL introduces logic to minimize the background draw rect, and avoid
unnecessary tiling for opaque borders.

Also, because it is mostly used as an offset, replace box_outset with
box_offset in BackgroundImageGeometry::Calculate.

BUG=676636

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I6b7095036a197df104fb34422d33943f5901aa27
Reviewed-on: https://chromium-review.googlesource.com/809229
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522133}