Add visualbench option for nvpr stencil samples

Add visualbench option for nvpr stencil samples

Repurposes the --nvpr flag as an int that says how many stencil samples
to use, or disables nvpr with a value of zero.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/60795d99fff8caedc2e82bebcfa8bd4fa07231b7

[joshualitt, 2015-11-13 15:19:27]

joshualitt@google.com changed reviewers:
+ joshualitt@google.com

[joshualitt, 2015-11-13 15:19:28]

lgtm

[Chris Dalton, 2015-11-13 15:54:57]

The CQ bit was checked by cdalton@nvidia.com

[commit-bot: I haz the power, 2015-11-13 15:55:00]

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

[commit-bot: I haz the power, 2015-11-13 15:55:01]

This CL has an open dependency (Issue 1441073005 Patch 1). Please resolve the
dependency and try again.

[Chris Dalton, 2015-11-13 16:59:32]

The CQ bit was checked by cdalton@nvidia.com

[commit-bot: I haz the power, 2015-11-13 16:59:40]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1444543002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1444543002/1

[commit-bot: I haz the power, 2015-11-13 17:10:34]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/60795d99fff8caedc2e82bebcfa8bd4fa07231b7