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This CL fixes two bugs with background-attachment:fixed computation:

1. Since CL 827671 we switch root layer in PaintInfo whenever the
current transform node changes. This has a side effect that the painting
root layer is no longer guaranteed to be a containing block ancestor of
the current layout object, and AccumulatedScrollOffsetForFixedBackground()
may skip over the painting root all the way to the root. This CL adds
the skipping check to ensure we only collect scroll offsets up to the
painting root.

2. FixedAttachmentPositioningArea() converts the viewport rect to the
space of the current paint context. Prior to SPv175 the border box space
of the painting root is always equivalent to the space of the current
paint context, so FixedAttachmentPositioningArea() did just that. In
SPv175 the painting root may not always create a paint offset
translation in case of a squashed layer, thus can induce an offset from
its transform node. This CL ensures the paint offset taken into account.

BUG=529963,849854

Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: Ib2f84a619235ab4ec6a9bfc695b57aae9d41b808
Reviewed-on: https://chromium-review.googlesource.com/1098313
Commit-Queue: Tien-Ren Chen <trchen@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567030}