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The LayoutBox associated with a block node will *normally* be the
containing block (on the legacy side) of its children (on the NG side),
but this isn't an invariant. Multicol and list item markers are
two examples.

Remove the kembo DCHECK, since it seems impossible to come up with a
complete list of exceptions to the rule (that the node's box should be
the containing block of the child nodes' boxes). Instead always get the
right containing block from legacy layout. Don't use that of the node.

With the DCHECK removed, a lot of tests stop crashing, and some just
start passing instead. One known effect of this change is that floats
that are direct children of multicol containers are painted correctly.
They used to be added to the float list of the multicol container,
rather than that of the flow thread, which caused two problems: paint
order (regular blocks got painted on top of them), and that they
appeared unfragmented (they just overflowed the multicol container
instead). Added a test for that.

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: I1ee926c39fbedfd52d87e9cf790996909914efb2
Reviewed-on: https://chromium-review.googlesource.com/960027
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Aleks Totic <atotic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543065}