Implement unset value handling

Implement unset value handling

This patch accepts unset as a valid CSS value and creates
a CSSUnsetValue object for it. Then when resolving it the
rules stated in below link are implemented:

http://dev.w3.org/csswg/css-cascade/#valuedef-unset

Add a test for the new behavior.

BUG=431689
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=186604

[rwlbuis, 2014-12-03 20:00:45]

Patchset #2 (id:20001) has been deleted

[rwlbuis, 2014-12-04 20:44:17]

rob.buis@samsung.com changed reviewers:
+ timloh@chromium.org

[rwlbuis, 2014-12-04 20:44:17]

PTAL. I had a hard time coming up with a generalized test for unset so I just
tested a few properties.

[Timothy Loh, 2014-12-05 00:08:21]

On 2014/12/04 20:44:17, rwlbuis wrote:
> PTAL. I had a hard time coming up with a generalized test for unset so I just
> tested a few properties.

lgtm, this seems fine to me.

[Timothy Loh, 2014-12-05 00:11:46]

On 2014/12/05 00:08:21, Timothy Loh wrote:
> On 2014/12/04 20:44:17, rwlbuis wrote:
> > PTAL. I had a hard time coming up with a generalized test for unset so I
just
> > tested a few properties.
> 
> lgtm, this seems fine to me.

Can we remove the special case in
StylePropertySerializer::StylePropertySetForSerializer::propertyAt now? :)

[rwlbuis, 2014-12-05 15:25:55]

The CQ bit was checked by rob.buis@samsung.com

[commit-bot: I haz the power, 2014-12-05 15:26:43]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/775153002/80001

[rwlbuis, 2014-12-05 16:27:36]

On 2014/12/05 00:11:46, Timothy Loh wrote:
> On 2014/12/05 00:08:21, Timothy Loh wrote:
> > On 2014/12/04 20:44:17, rwlbuis wrote:
> > > PTAL. I had a hard time coming up with a generalized test for unset so I
> just
> > > tested a few properties.
> > 
> > lgtm, this seems fine to me.
> 
> Can we remove the special case in
> StylePropertySerializer::StylePropertySetForSerializer::propertyAt now? :)

I'll remove it and see if everything remains green :)

[commit-bot: I haz the power, 2014-12-05 16:46:51]

Committed patchset #4 (id:80001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=186604

[Ilya Sherman, 2014-12-05 22:33:53]

A revert of this CL (patchset #4 id:80001) has been created in
https://codereview.chromium.org/782853002/ by isherman@chromium.org.

The reason for reverting is: This broke the Linux Oilpan Builder.

Logs:
http://build.chromium.org/p/chromium.perf/builders/Linux%20Oilpan%20Builder/b...

BUG=439604.