Mark column sets as invalid and kill the interval tree when column sets are removed.

Mark column sets as invalid and kill the interval tree when column sets are removed.

This makes the multicol code more fault-tolerant.

If there are bugs somewhere in the engine that cause us to miss layout of
objects marked for layout, the multicol code could return dead objects from the
column set interval tree. Better just delete the interval tree right away when
column sets are removed, instead of keeping dead objects around. The tree will
be rebuilt during layout.

To clarify: This CL doesn't really fix bug 498664, but at least we won't crash
because of accessing dead objects anymore (only assert in
LayoutObject::assertLaidOut() and other places, like before).

BUG=498664
R=dsinclair@chromium.org,jchaffraix@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=196876

[mstensho (USE GERRIT), 2015-06-10 10:46:08]

No test in this CL, since the problem that this CL fixes depends on other bugs
in the engine. The multicol test attached in bug 498664 will still assert (even
more so than before, actually, since column sets now will be appropriately
marked as invalidated), but at least it won't crash for real, since we won't be
working on dead objects anymore.

[dsinclair, 2015-06-10 14:22:23]

The CQ bit was checked by dsinclair@chromium.org

[dsinclair, 2015-06-10 14:22:24]

lgtm

[commit-bot: I haz the power, 2015-06-10 14:22:30]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1163423007/1

[commit-bot: I haz the power, 2015-06-10 15:16:11]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=196876