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

third_party/libpng/libpng.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.
 
 {
   'variables': {
-    'use_system_libpng%': 0,
+    'conditions': [
+      [ 'OS=="linux"', {
+        # Link to system .so since we already use it due to GTK.
+        'use_system_libpng%': 1,
+      }, {  # OS!="linux"
+        'use_system_libpng%': 0,
+      }],
+    ],
   },
   'conditions': [
     ['use_system_libpng==0', {
       'targets': [
         {
           'target_name': 'libpng',
           'type': '<(library)',
           'dependencies': [
             '../zlib/zlib.gyp:zlib',
           ],
@@ skipping 69 matching lines @@
       ],
     }],
   ],
 }
 
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