When changing a layout object from in-flow to out-of-flow positioned, we
used to just remove it from the flow thread, risking that there'd no
longer be a column set to associate it with. However, an out-of-flow
positioned descendant may be contained by something that's inside the
flow thread, e.g. if the containing block of an absolutely positioned
object is a relatively positioned object, and that relatively positioned
object is contained by the flow thread.

Since it's hard to detect what the new containing block of an object is
going to be before it has actually gone out of flow, we'll still remove
it from the flow thread, but we'll now detect that we need to re-insert
it when computed style has updated.

Bug: 827424
Change-Id: I413348b0d3ecd0c4b5051e6e9d2a4526863bef60
Reviewed-on: https://chromium-review.googlesource.com/995439
Reviewed-by: Emil A Eklund <eae@chromium.org>
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548354}