Move self-collapse checking to LayoutBlockFlow and cache it completely.
Only block containers (that's LayoutBlockFlow in Blinquese) can have adjoining
top and bottom margins, since all other LayoutBlock derivates establish some
kind of formatting context (table, flexbox, etc.).
Also cache self-collapsedness completely. Previously we only used the cached
result if we had previously found that the entire subtree is self-collapsing.
The new approach eliminates the need for "mutable" too.
BUG=302024
Review-Url: https://codereview.chromium.org/1933153002
Cr-Commit-Position: refs/heads/master@{#390729}
4 files changed
