Introduce LayoutBox::paginationBreakability().

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

Committed: https://crrev.com/67fa0a7acb5a84802c2da62aec914697f13e9e72
Cr-Commit-Position: refs/heads/master@{#357559}

[leviw_travelin_and_unemployed, 2015-11-03 17:09:16]

The CQ bit was checked by leviw@chromium.org

[leviw_travelin_and_unemployed, 2015-11-03 17:09:16]

lgtm

[commit-bot: I haz the power, 2015-11-03 17:10:03]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1426673005/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1426673005/1

[commit-bot: I haz the power, 2015-11-03 18:21:54]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-11-03 18:23:57]

Patchset 1 (id:??) landed as
https://crrev.com/67fa0a7acb5a84802c2da62aec914697f13e9e72
Cr-Commit-Position: refs/heads/master@{#357559}