Resolve :root and table-cell currentcolor using background box's style

In BackgroundImageGeometry::ImageStyle we'd always return the computed
style for |box_| (the box for which we're painting), and thus use that
to resolve currentcolor, although in some cases (for :root and table-
cells that get their backgrounds from the row or column) we ought be
using the style from the element that the background originated from.

Bug: 848860
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I9113957889a3f091d3fe71830c0805a4064734b5
Reviewed-on: https://chromium-review.googlesource.com/1203373
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#588536}
7 files changed