Revert of Revert of Implement unset value handling

Revert of Revert of Implement unset value handling (patchset #1 id:1 of https://codereview.chromium.org/782853002/)

Reason for revert:
A build fix has landed, so the revert is no longer necessary.

Original issue's description:
> Revert of Implement unset value handling (patchset #4 id:80001 of https://codereview.chromium.org/775153002/)
> 
> Reason for revert:
> This broke the Linux Oilpan Builder.
> 
> Logs: http://build.chromium.org/p/chromium.perf/builders/Linux%20Oilpan%20Builder/builds/13454/steps/compile/logs/stdio#error1
> 
> BUG=439604
> 
> Original issue's description:
> > 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
> 
> TBR=timloh@chromium.org,rob.buis@samsung.com
> NOTREECHECKS=true
> NOTRY=true
> BUG=431689
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=186627

TBR=timloh@chromium.org,rob.buis@samsung.com
NOTREECHECKS=true
NOTRY=true
BUG=439604
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=186637

[Ilya Sherman, 2014-12-06 01:16:27]

Created Revert of Revert of Implement unset value handling

[commit-bot: I haz the power, 2014-12-06 01:17:32]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/773483003/1

[commit-bot: I haz the power, 2014-12-06 01:18:14]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=186637