Revert "remove -Wno-header-guard workaround for old versions of NSS"

Revert "remove -Wno-header-guard workaround for old versions of NSS"

This reverts commit 86ebf48be903b226d068b08efa16fe3725babb8c.

Build failures:
  http://build.chromium.org/p/chromium.chromiumos/builders/Linux%20ChromiumOS%20Builder/builds/20027
  http://build.chromium.org/p/chromium.webkit/builders/GPU%20Linux%20Builder%20%28dbg%29/builds/31334

Original issue's description:
> remove -Wno-header-guard workaround for old versions of NSS
>
> I think it's safe to re-enable this warning now.  The bug was
> fixed upstream in 3.15.1 and there have been several NSS upgrades since then.
>
> BUG=255201
> Committed: https://crrev.com/86ebf48be903b226d068b08efa16fe3725babb8c
> Cr-Commit-Position: refs/heads/master@{#321711}

TBR=mostynb@opera.com,thakis@chromium.org
BUG=255201
NOTREECHECKS=true
NOTRY=true

Committed: https://crrev.com/7e9611ced1805c43af09fccc03367313e5253ca9
Cr-Commit-Position: refs/heads/master@{#321712}

[sof, 2015-03-22 11:23:36]

sigbjornf@opera.com changed reviewers:
+ thakis@chromium.org

[sof, 2015-03-22 11:23:37]

ptal

[sof, 2015-03-22 11:23:42]

The CQ bit was checked by sigbjornf@opera.com

[commit-bot: I haz the power, 2015-03-22 11:24:06]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1024343002/1

[commit-bot: I haz the power, 2015-03-22 11:24:38]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-22 11:25:29]

Patchset 1 (id:??) landed as
https://crrev.com/7e9611ced1805c43af09fccc03367313e5253ca9
Cr-Commit-Position: refs/heads/master@{#321712}

[Mostyn Bramley-Moore, 2015-03-22 17:31:28]

LGTM.  Thanks for reverting this.