Add assertion/crash test for flexbox with relpos with scrollable with abspos.

Add assertion/crash test for flexbox with relpos with scrollable with abspos.

This test (attached to bug 498969) used to assert, but got fixed by accident by
the fix for bug 498770. Better add the test, in case it should regress.

BUG=498969
R=cbiesinger@chromium.org

Committed: https://crrev.com/169f80b557c2d4c83479c058073b2225270ae4da
git-svn-id: svn://svn.chromium.org/blink/trunk@199646 bbb929c8-8fbe-4397-9dbb-9b2b20218538

[cbiesinger, 2015-07-28 15:04:17]

You'd think that fast/flexbox is for the flexbox tests. However, they are
actually for the deprecated flexbox (display: -webkit-box).

lgtm if you move this to css3/flexbox.

[mstensho (USE GERRIT), 2015-07-29 08:12:21]

On 2015/07/28 15:04:17, cbiesinger wrote:
> You'd think that fast/flexbox is for the flexbox tests. However, they are
> actually for the deprecated flexbox (display: -webkit-box).
> 
> lgtm if you move this to css3/flexbox.

Done.

[mstensho (USE GERRIT), 2015-07-29 08:12:34]

The CQ bit was checked by mstensho@opera.com

[mstensho (USE GERRIT), 2015-07-29 08:12:35]

The patchset sent to the CQ was uploaded after l-g-t-m from
cbiesinger@chromium.org
Link to the patchset: https://codereview.chromium.org/1261463005/#ps20001
(title: "Moved test to the correct directory.")

[commit-bot: I haz the power, 2015-07-29 08:12:44]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1261463005/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1261463005/20001

[commit-bot: I haz the power, 2015-07-29 09:25:59]

Committed patchset #2 (id:20001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=199646

[commit-bot: I haz the power, 2015-09-23 11:53:04]

Patchset 2 (id:??) landed as
https://crrev.com/169f80b557c2d4c83479c058073b2225270ae4da