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This rids us of some crufty code, and also fixes bugs. The approach was also
broken for multicol, because we started UpdatePaginationRecursive() in the
middle of the tree without looking for a containing flow thread (pagination
layer). This would result in the new layer erroneously not becoming paginated.

Also had to update a unit test, to satisfy its requirement that the style change
won't trigger layout.

BUG=616596

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I50ed61a7174e360259b7b786bab01cf74616fc32
Reviewed-on: https://chromium-review.googlesource.com/542915
Commit-Queue: Morten Stenshorne <mstensho@opera.com>
Reviewed-by: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488768}