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

Review URL: https://codereview.chromium.org/1024343002

Cr-Original-Commit-Position: refs/heads/master@{#321712}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 7e9611ced1805c43af09fccc03367313e5253ca9

linux/system.gyp

@@ -1204,6 +1204,15 @@
                 ],
               },
             }],
+            ['use_openssl==0 and clang==1', {
+              'direct_dependent_settings': {
+                'cflags': [
+                  # There is a broken header guard in /usr/include/nss/secmod.h:
+                  # https://bugzilla.mozilla.org/show_bug.cgi?id=884072
+                  '-Wno-header-guard',
+                ],
+              },
+            }],
           ]
         }],
       ],