[New Multicolumn] Don't return the wrong flow thread from locateFlowThreadContainingBlock().

[New Multicolumn] Don't return the wrong flow thread from locateFlowThreadContainingBlock().

LayoutListItem::updateMarkerLocationAndInvalidateWidth() does nasty
things (inserts layout objects at "random" places in the tree during
layout), so we need to temporarily reset the current flow thread in
LayoutState, or we might end up locating the wrong one in
LayoutObject::locateFlowThreadContainingBlock(). Need this until
crbug.com/370461 gets fixed.

This makes fast/lists/list-inside-columns-crash.html pass with the new multicol
implementation, but since that test is still running with the old multicol
implementation, I added a simplified tailored test that's actually run.

BUG=334335, 460251
R=dsinclair@chromium.org,jchaffraix@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=192423

[Julien - ping for review, 2015-03-23 15:20:49]

I think we should finish https://codereview.chromium.org/778003003 instead of
this work-around.

Dan can comment on the status of the change.

[mstensho (USE GERRIT), 2015-03-23 22:51:50]

On 2015/03/23 15:20:49, Julien Chaffraix - PST wrote:
> I think we should finish https://codereview.chromium.org/778003003 instead of
> this work-around.

That would be ideal, but this is one of the (two) last things preventing us from
enabling the new multicol implementation in testing.

So unless fixing list items is right around the corner, I hope that we can live
with this work-around in the meantime.

[dsinclair, 2015-03-24 00:10:11]

That change is going to take more time to land. I think landing this work around
is probably a good step for now. The code will get killed when I land the other
change, but having the extra tests is a good thing.

lgtm.

[mstensho (USE GERRIT), 2015-03-24 08:38:31]

The CQ bit was checked by mstensho@opera.com

[commit-bot: I haz the power, 2015-03-24 08:38:43]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1021143004/1

[commit-bot: I haz the power, 2015-03-24 10:03:59]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=192423