Make sure the 'use_system_*' variables are actually visible in the relevant gyp files.

build/linux/system.gyp

 # Copyright (c) 2009 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 {
   'conditions': [
     ['sysroot!=""', {
       'variables': {
         'pkg-config': './pkg-config-wrapper "<(sysroot)"',
       },
     }, {
       'variables': {
         'pkg-config': 'pkg-config'
       },
     }],
+    [ 'OS=="linux"', {
+      'variables': {
+        # We use our own copy of libssl, although we still need to link against
+        # the rest of NSS.
+        'use_system_ssl%': 0,
+      },
+    }, {  # OS!="linux"
+      'variables': {
+        'use_system_ssl%': 1,
+      },
+    }],
   ],
 
-  'variables': {
-    'use_system_ssl%': 1,
-  },
 
   'targets': [
     {
       'target_name': 'gtk',
       'type': 'settings',
       'conditions': [
         ['_toolset=="target"', {
           'direct_dependent_settings': {
             'cflags': [
               '<!@(<(pkg-config) --cflags gtk+-2.0 gthread-2.0)',
@@ skipping 225 matching lines @@
 #       },
 #     },
   ],
 }
 
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