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This reverts commit e06da142.

Reason for revert: Was not the root cause of test failures.

Original change's description:
> Revert "Fix background painting on scrolling contents layer with foregrounds"
> 
> This reverts commit 7834ee21.
> 
> Reason for revert: Suspected as causing failing tests in 874162
> 
> Original change's description:
> > Fix background painting on scrolling contents layer with foregrounds
> > 
> > This patch removes a negative z-index children check from
> > PaintLayer::GetBackgroundPaintLocation. This lets backgrounds paint in
> > the scrolling contents layer in the presence of negative z-index
> > children, fixing the painting of background-attachment: local. This
> > patch also removes a DCHECK that was hit when a foreground layer and
> > kBackgroundPaintInScrollingContents coexisted. This DCHECK[1] could be
> > hit for root layers without the functional change in this patch. As the
> > test in this patch shows, kBackgroundPaintInScrollingContents and
> > foreground layers work together, so this patch makes non-root-layers use
> > both, and removes the DCHECK.
> > 
> > What are foreground layers? A foreground layer is created if a stacked
> > PaintLayer has composited negative z-index children that should wedge
> > between the layer's background and the layer's contents. The foreground
> > layer will have the normal-flow contents in this case. See the comment
> > above foreground_layer_ in composited_layer_mapping.h.
> > 
> > [1] DCHECK originally added in: https://crrev.com/9a2cdc8d96d. Reading
> > the comments, I think the DCHECK was intended to ensure the background
> > would not be painted into the foreground layer.
> > 
> > Bug: 861948
> > Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
> > Change-Id: I7409cbe9ac61fed444856df342d0707d6babab09
> > Reviewed-on: https://chromium-review.googlesource.com/1173532
> > Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
> > Commit-Queue: Philip Rogers <pdr@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#584802}
> 
> TBR=pdr@chromium.org,chrishtr@chromium.org
> 
> Change-Id: Ifd22a963dbd5ce288dfb31ab1e8ac41ebe63062d
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: 861948
> Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
> Reviewed-on: https://chromium-review.googlesource.com/1184109
> Reviewed-by: Philip Rogers <pdr@chromium.org>
> Commit-Queue: Philip Rogers <pdr@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#584895}

TBR=pdr@chromium.org,chrishtr@chromium.org

Change-Id: I569313a9d5e1dd0b79e541cc3e3ae8816c4c3e05
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 861948
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Reviewed-on: https://chromium-review.googlesource.com/1184181Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584911}