Fix cert DB usage in PolicyCertVerifier test.

Fix cert DB usage in PolicyCertVerifier test.

- Replaces ScopedTestNSSDB by ScopedTestNSSChromeOSUser, which is the choice for ChromeOS.
- Removes the deprecated net::NSSCertDatabase::GetInstance() call.
- Changes the build target from brower_test to unit_test, as originally intended.

BUG=329735, 210525
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=284112

[pneubeck (no reviews), 2014-07-18 12:29:15]

Julian, ptal.

Jochen, need your lgtm for the gyp changes.

[jochen (gone - plz use gerrit), 2014-07-18 12:30:45]

gyp lgtm

[pastarmovj, 2014-07-18 12:38:01]

lgtm

[pneubeck (no reviews), 2014-07-18 12:42:27]

The CQ bit was checked by pneubeck@chromium.org

[commit-bot: I haz the power, 2014-07-18 12:43:06]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/pneubeck@chromium.org/407443008/1

[commit-bot: I haz the power, 2014-07-18 14:29:16]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  win_chromium_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/win_chromium_rel/buil...)

[commit-bot: I haz the power, 2014-07-18 16:11:55]

Change committed as 284112