GitLab

Re-land https://codereview.chromium.org/2531953002 and
https://codereview.chromium.org/2547933003.

When a negative margin top pushes a block back up into its previous siblings
we need to check for any floats in those siblings it now needs to avoid or clear.

Previously we were just looking at its neighbour, we need to keep looking until
we reach a sibling that we don't overlap.

Fix the performance regression on mossiella.com in page_cycler_v2.tough_layout_cases
by only looking for floats if margin collapsing has actually moved the child up.

BUG=671645, 670325, 666487

Review-Url: https://codereview.chromium.org/2568643002
Cr-Commit-Position: refs/heads/master@{#437951}