Locate the correct flow thread for spanners and out-of-flow objects.

LayoutFlowThread::LocateFlowThreadContainingBlockOf() used to just
give up if the nearest ancestor flow thread of some object wasn't the
containing flow thread.

The machinery that maintains the special-objects
(LayoutMultiColumnSet, LayoutMulticolumnSpannerPlaceholder) for
multicol still needs to ignore out-of-flow objects and spanners,
though, so I moved the check for this from
LocateFlowThreadContainingBlockOf() to
FlowThreadDescendantWasInserted() and
FlowThreadDescendantWillBeRemoved().

Checking on the paint side whether page logical height is known is
no longer necessary.

Bug: 757947
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: Ief0492e9de52b01b08c3bc0318cb3c8abe67ccd3
Reviewed-on: https://chromium-review.googlesource.com/632057
Reviewed-by: Emil A Eklund <eae@chromium.org>
Commit-Queue: Morten Stenshorne <mstensho@opera.com>
Cr-Commit-Position: refs/heads/master@{#497437}