Add support for z-index: auto to AnimatableValues

Add support for z-index: auto to AnimatableValues

AnimatableValues did not support the auto keyword for z-index. This resulted
in assertion failures when checking the new Interpolation pipeline against
the old AnimatableValue pipeline.
This patch adds support for this and resolves the assertion failure.

BUG=497730
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=197002

[alancutter (OOO until 2018), 2015-06-09 07:06:01]

alancutter@chromium.org changed reviewers:
+ dstockwell@chromium.org

[dstockwell, 2015-06-09 07:50:50]

Looks like there's a similar bug in CSSPropertyEquality.

[alancutter (OOO until 2018), 2015-06-10 08:23:13]

On 2015/06/09 at 07:50:50, dstockwell wrote:
> Looks like there's a similar bug in CSSPropertyEquality.

Fixed.

[dstockwell, 2015-06-10 08:33:09]

The CQ bit was checked by dstockwell@chromium.org

[dstockwell, 2015-06-10 08:33:09]

lgtm

[commit-bot: I haz the power, 2015-06-10 08:33:26]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1172583002/20001

[commit-bot: I haz the power, 2015-06-10 09:12:11]

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

[commit-bot: I haz the power, 2015-06-10 09:12:12]

Try jobs failed on following builders:
  linux_blink_rel on tryserver.blink (JOB_FAILED,
http://build.chromium.org/p/tryserver.blink/builders/linux_blink_rel/builds/6...)

[alancutter (OOO until 2018), 2015-06-12 01:45:27]

The CQ bit was checked by alancutter@chromium.org

[alancutter (OOO until 2018), 2015-06-12 01:45:27]

Removed zIndex() getter assertion as it was getting hit in other contexts, filed
https://code.google.com/p/chromium/issues/detail?id=499651 for adding it back
in.

[alancutter (OOO until 2018), 2015-06-12 01:45:28]

The patchset sent to the CQ was uploaded after l-g-t-m from
dstockwell@chromium.org
Link to the patchset: https://codereview.chromium.org/1172583002/#ps40001
(title: "Remove assert")

[commit-bot: I haz the power, 2015-06-12 01:45:58]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1172583002/40001

[commit-bot: I haz the power, 2015-06-12 03:12:04]

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

[commit-bot: I haz the power, 2015-06-12 03:12:05]

Try jobs failed on following builders:
  linux_blink_rel on tryserver.blink (JOB_FAILED,
http://build.chromium.org/p/tryserver.blink/builders/linux_blink_rel/builds/6...)

[alancutter (OOO until 2018), 2015-06-12 04:41:45]

The CQ bit was checked by alancutter@chromium.org

[alancutter (OOO until 2018), 2015-06-12 04:41:45]

The patchset sent to the CQ was uploaded after l-g-t-m from
dstockwell@chromium.org
Link to the patchset: https://codereview.chromium.org/1172583002/#ps60001
(title: "Remove dumb")

[commit-bot: I haz the power, 2015-06-12 04:41:52]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1172583002/60001

[commit-bot: I haz the power, 2015-06-12 06:00:29]

Committed patchset #4 (id:60001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=197002