Introduce LayoutBox::paginationBreakability().
The engine actually cares about three states of column / page breakability
inside an object:
1. Breakable. Look inside for possible break points.
2. Unbreakable. Breaking inside would pretty much guarantee ugly layout.
3. Preferable not to break (break-inside:avoid).
LayoutBox::isUnsplittableForPagination() returned true for #2, and false for #1
and #3.
The only one that cares about #3 at the moment is adjustForUnsplittableChild()
in LayoutBlockFlow, which handled #3 on its own after checking with
isUnsplittableForPagination(), but we're soon going to have to care about it at
other places too, so let's introduce a method that can tell us.
R=leviw@chromium.org
Review URL: https://codereview.chromium.org/1426673005
Cr-Commit-Position: refs/heads/master@{#357559}
7 files changed
