Rename png_ functions conflicting with chrome.

pdfium.gyp

 # Copyright 2015 PDFium 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': {
     # TODO(thakis): Enable this, pdfium:29
     #'chromium_code': 1,
     'pdf_use_skia%': 0,
     'pdf_enable_v8%': 1,
     'pdf_enable_xfa%': 0, # Set to 1 by standalone.gypi in a standalone build.
     'conditions': [
       ['OS=="linux"', {
         'bundle_freetype%': 0,
       }, {  # On Android there's no system FreeType. On Windows and Mac, only a
             # few methods are used from it.
         'bundle_freetype%': 1,
       }],
     ],
   },
   'target_defaults': {
     'defines' : [
       'OPJ_STATIC',
       'PNG_PREFIX',
-      'PNGPREFIX_H',
       'PNG_USE_READ_MACROS',
       'V8_DEPRECATION_WARNINGS',
       '_CRT_SECURE_NO_WARNINGS',
     ],
     'include_dirs': [
       # This is implicit in GN.
       '<(DEPTH)',
       '.',
       'third_party/freetype/include',
       'third_party/freetype/include/freetype',
@@ skipping 840 matching lines @@
             'fpdfsdk/include/fpdfxfa/fpdfxfa_app.h',
             'fpdfsdk/include/fpdfxfa/fpdfxfa_doc.h',
             'fpdfsdk/include/fpdfxfa/fpdfxfa_page.h',
             'fpdfsdk/include/fpdfxfa/fpdfxfa_util.h',
           ],
         },
       ]
     }],
   ]
 }