[New Multicolumn] Simplified layout also needs to mark spanner placeholders for layout.

[New Multicolumn] Simplified layout also needs to mark spanner placeholders for layout.

If the actual column-span:all object needs layout, so does the placeholder.
There was code to ensure this for normal layout, but not for simplified layout
(when we just want to drill into the tree and locate the containing block of
some absolutely positioned object.

BUG=483976
R=dsinclair@chromium.org,jchaffraix@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=194861

[dsinclair, 2015-05-04 13:03:35]

The CQ bit was checked by dsinclair@chromium.org

[dsinclair, 2015-05-04 13:03:36]

lgtm

[commit-bot: I haz the power, 2015-05-04 13:03:44]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1120313003/1

[commit-bot: I haz the power, 2015-05-04 13:06:38]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=194861