Rewrite fast/multicol/shadow-breaking.html as a check-layout test.

Rewrite fast/multicol/shadow-breaking.html as a check-layout test.

The old / current multicol implementation had imperfect column balancing in the
old test, so writing a reftest would be tricky, but anyway: it's not obvious
that our current behavior (both implementations) where we include the shadows
in the columns, rather than letting them overflow, is correct (as a matter of
fact, it's most likely wrong). So let's just test what really needs to work:
that a shadow doesn't incorrectly push the next line down.

BUG=386567
R=andersr@opera.com

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=190085

[andersr, 2015-02-12 22:07:55]

lgtm

[mstensho (USE GERRIT), 2015-02-12 22:21:23]

The CQ bit was checked by mstensho@opera.com

[commit-bot: I haz the power, 2015-02-12 22:22:35]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/910313003/1

[commit-bot: I haz the power, 2015-02-12 22:27:28]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=190085