Fix visual overflow for LayoutBoxes with numeric border image outsets

Fix visual overflow for LayoutBoxes with numeric border image outsets

When computing the visual overflow in LayoutBox::addVisualEffectOverflow
we would skip adding border image outset overflow because
ComputedStyle::hasVisualOverflowingEffect would return false for
non-zero numeric outsets. This was caused by a typo in
BorderImageLength::isZero (missing '!' before m_number).

TEST=fast/borders/border-image-outset.html with slimming paint and
SlimmingPaintStrictCullRectClipping enabled.

BUG=496319
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=196584

[pdr., 2015-06-05 05:28:22]

pdr@chromium.org changed reviewers:
+ chrishtr@chromium.org, wangxianzhu@chromium.org

[pdr., 2015-06-05 06:56:59]

The CQ bit was checked by pdr@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-06-05 06:57:04]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1169623002/1

[commit-bot: I haz the power, 2015-06-05 07:56:31]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-06-05 07:56:32]

Dry run: This issue passed the CQ dry run.

[Xianzhu, 2015-06-05 15:55:01]

The CQ bit was checked by wangxianzhu@chromium.org

[Xianzhu, 2015-06-05 15:55:02]

lgtm

[commit-bot: I haz the power, 2015-06-05 15:55:21]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1169623002/1

[commit-bot: I haz the power, 2015-06-05 16:20:10]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=196584