[New Multicolumn] Disallow spanners inside floats and absolutely positioned boxes.

[New Multicolumn] Disallow spanners inside floats and absolutely positioned boxes.

Since the logical top of a float or absolutely positioned box isn't (guaranteed
to be) known until after layout, we get problems determining the logical height
of the flow thread portion preceding the spanner, which in turn means that
we'll get column balancing wrong. The usefulness of spanners inside floats or
absolutely positioned boxes is questionable, so just disallow spanners inside
such boxes for now.

BUG=479085
R=dsinclair@chromium.org,jchaffraix@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=194152

[dsinclair, 2015-04-21 18:39:04]

The CQ bit was checked by dsinclair@chromium.org

[dsinclair, 2015-04-21 18:39:04]

lgtm

[commit-bot: I haz the power, 2015-04-21 18:39:53]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1100823002/1

[commit-bot: I haz the power, 2015-04-21 18:57:38]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=194152