Fix performance regression in png encoding caused by libpng update

third_party/libpng/libpng.gyp

 # Copyright (c) 2012 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.
 
 {
   'targets': [
     {
       'target_name': 'libpng',
       'dependencies': [
         '../zlib/zlib.gyp:zlib',
@@ skipping 29 matching lines @@
         'pngwutil.c',
       ],
       'direct_dependent_settings': {
         'include_dirs': [
           '.',
         ],
       },
       'export_dependent_settings': [
         '../zlib/zlib.gyp:zlib',
       ],
-      # TODO(jschuh): http://crbug.com/167187
-      'msvs_disabled_warnings': [ 4267 ],
+      'msvs_disabled_warnings': [
+        4267, # TODO(jschuh): http://crbug.com/167187
+        4146, # Unary minus applied to unsigned type.
+      ],
       'conditions': [
         # Disable ARM optimizations on IOS.  Can't find a way to get gyp to even try
         # to compile the optimization files.  This works fine on GN.
         [ 'OS=="ios"', {
           'defines': [
             'PNG_ARM_NEON_OPT=0',
           ],
         }],
 
         # SSE optimizations
@@ skipping 38 matching lines @@
             ],
           },
         }],
         ['OS=="android"', {
           'toolsets': ['target', 'host'],
         }],
       ],
     },
   ]
 }