Revert of Implement unset value handling

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

[Ilya Sherman, 2014-12-05 22:33:54]

Created Revert of Implement unset value handling

[commit-bot: I haz the power, 2014-12-05 22:34:04]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/782853002/1

[commit-bot: I haz the power, 2014-12-05 22:34:50]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=186627

[Timothy Loh, 2014-12-06 00:42:44]

On 2014/12/05 22:34:50, I haz the power (commit-bot) wrote:
> Committed patchset #1 (id:1) as
> https://src.chromium.org/viewvc/blink?view=rev&revision=186627

I thought we don't revert for oilpan breakages? :/

[Ilya Sherman, 2014-12-06 00:45:07]

On 2014/12/06 00:42:44, Timothy Loh wrote:
> On 2014/12/05 22:34:50, I haz the power (commit-bot) wrote:
> > Committed patchset #1 (id:1) as
> > https://src.chromium.org/viewvc/blink?view=rev&revision=186627
> 
> I thought we don't revert for oilpan breakages? :/

Sorry, I'm not familiar with the specific policy, and the Blink gardener wasn't
around.  Not knowing the policy, it seems to me like broken compile merits a
revert though, even if broken tests might not.  But, feel free to re-land if the
revert was in error.  Thanks.

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

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/773483003/ by isherman@chromium.org.

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