Crash test for 400997.

Crash test for 400997.

Scrollbars can be kept alive referenced from the EventHandler after they
have been detached from their ScrollableAreas (through
m_lastScrollbarUnderMouse and m_scrollbarHandlingScrollGesture).
EventHandler may call setHoveredPart and setPressedPart on custom scroll-
bars (LayoutScrollbar) on mouse moves after these bars have been detached
from their scrollable areas. Those methods recalculate style for custom
scrollbar pseudo elements. That caused a crash in SelectorChecker for
the :corner-present pseudo class because it used the scrollableArea()
from Scrollbar without null-checking it.

The null-check was added as a speculative fix in r191244. This CL adds a
test which causes that crasher.

R=keishi@chromium.org,tkent@chromium.org
BUG=400997
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=191278

[rune, 2015-03-04 09:47:30]

PTAL

[keishi, 2015-03-04 13:18:24]

Thanks! LGTM.

[rune, 2015-03-04 13:20:58]

The CQ bit was checked by rune@opera.com

[commit-bot: I haz the power, 2015-03-04 13:22:12]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/978753002/1

[commit-bot: I haz the power, 2015-03-04 13:25:05]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=191278