Revert of Revert of cc: Default threaded GPU rasterization to ON.

Revert of Revert of cc: Default threaded GPU rasterization to ON. (patchset #1 id:1 of https://codereview.chromium.org/936803002/)

Reason for revert:
Known issue with this device.  Workaround is incoming crrev.com/896303004/.

Original issue's description:
> Revert of cc: Default threaded GPU rasterization to ON. (patchset #1 id:1 of https://codereview.chromium.org/927463004/)
>
> Reason for revert:
> Breaks compositor rendering on Nexus 6: crbug.com/459557
>
> Original issue's description:
> > cc: Default threaded GPU rasterization to ON.
> >
> > BUG=454500
> >
> > Committed: https://crrev.com/96a23833e024783905297b60e7d86ca588ada45c
> > Cr-Commit-Position: refs/heads/master@{#316665}
>
> TBR=sievers@chromium.org,vmiura@chromium.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=454500
>
> Committed: https://crrev.com/2b0273be4aa28177d202942eadea5e8086f1ae79
> Cr-Commit-Position: refs/heads/master@{#316840}

TBR=sievers@chromium.org,skyostil@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=454500
Committed: https://crrev.com/8a54728bf7062805ec3ebe7fcdc32f21b96d7a2c
Cr-Commit-Position: refs/heads/master@{#316856}

[vmiura, 2015-02-18 17:50:10]

Created Revert of Revert of cc: Default threaded GPU rasterization to ON.

[vmiura, 2015-02-18 17:50:24]

The CQ bit was checked by vmiura@chromium.org

[commit-bot: I haz the power, 2015-02-18 17:50:55]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/932183002/1

[commit-bot: I haz the power, 2015-02-18 17:51:29]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-18 17:52:01]

Patchset 1 (id:??) landed as
https://crrev.com/8a54728bf7062805ec3ebe7fcdc32f21b96d7a2c
Cr-Commit-Position: refs/heads/master@{#316856}

[Sami, 2015-02-18 17:52:28]

On 2015/02/18 17:50:10, vmiura wrote:
> Created Revert of Revert of cc: Default threaded GPU rasterization to ON.

Might want to let the fix land first :)

[vmiura, 2015-02-18 19:19:55]

On 2015/02/18 17:52:28, Sami wrote:
> On 2015/02/18 17:50:10, vmiura wrote:
> > Created Revert of Revert of cc: Default threaded GPU rasterization to ON.
> 
> Might want to let the fix land first :)

Yeah, sorry I want to this to stay on so we can find any other issues.

The Nexus 6 bug was reproduced before Threaded GPU as well, although this made
it appear more easily.  crbug.com/457027.