[New Multicolumn] Disallow multicol on replaced content and fieldsets.

[New Multicolumn] Disallow multicol on replaced content and fieldsets.

It's meaningless to multicol replaced content. For fieldsets it might make
sense, though, but currently we cannot, because the
layoutSpecialExcludedChild() mechanism only allows one special child per object
(the flow thread and the legend would make for two).

layoutSpecialExcludedChild() was especially bad for a TEXTAREA with a
placeholder attribute when attempting to multicol it, because we would go ahead
and lay out the placeholder (during TEXTAREA layout), which would be a child of
the flow thread. So we would lay out the grandchild before laying out its
parent, causing assertion failures in
LayoutMultiColumnFlowThread::columnSetAtBlockOffset().

BUG=482362
R=dsinclair@chromium.org,jchaffraix@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=194657

[dsinclair, 2015-04-29 13:24:56]

The CQ bit was checked by dsinclair@chromium.org

[dsinclair, 2015-04-29 13:24:57]

lgtm

[commit-bot: I haz the power, 2015-04-29 13:25:08]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1110403002/1

[commit-bot: I haz the power, 2015-04-29 13:29:04]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=194657