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Instead of using one of the squashed layers as the starting point, use
the transformed ancestor, from which we have already computed an offset
to the squashing layer.

As a result, remove the squashing-prevention rule about non-translation
transforms.

Bug: 750433
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I4cf1fc5a2e34d7e40023d4c3481bb517c3b6e72f
Reviewed-on: https://chromium-review.googlesource.com/658335Reviewed-by: Tien-Ren Chen <trchen@chromium.org>
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501157}