Out-of-flow objects inside multicol may not establish column sets.

Out-of-flow objects inside multicol may not establish column sets.

Do a better job at ignoring out-of-flow objects that are not contained by
anything inside the multicol container. Previously we only made sure to skip
such objects at one specific location in flowThreadDescendantWasInserted(), but
this is really needed all over the place, so added utility functions for it.

Added an assertion mapDescendantToColumnSet() to catch such situations earlier
in the future.

BUG=492897
R=dsinclair@chromium.org,jchaffraix@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=196234

[dsinclair, 2015-06-01 15:17:03]

The CQ bit was checked by dsinclair@chromium.org

[dsinclair, 2015-06-01 15:17:04]

lgtm

[commit-bot: I haz the power, 2015-06-01 15:17:23]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1163603002/1

[commit-bot: I haz the power, 2015-06-01 16:28:13]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=196234