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Previously we calculated layout opportunities by shrinking the available
child space by the child's margins.

This resulted in layout opportunities which were too small, and would
push the new formatting context down to the next opportunity.

This logic now allows margins to "overhang" a layout opportunity.

At the same time I refactored a lot of the code, as we were
adding/subtracting margins for different calculations. Hopefully this
is more readable!

Bug: 1003193
Change-Id: I6c05b050332afe9880ca7d9a5b426b4b94fde3f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1808661
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#698954}