Rename png_ functions conflicting with chrome.

samples/samples.gyp

 # Copyright 2014 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': {
     'pdf_enable_v8%': 1,
     'pdf_enable_xfa%': 0,  # Set to 1 in standalone builds by standalone.gypi.
   },
   'target_defaults': {
     'defines' : [
       'PNG_PREFIX',
-      'PNGPREFIX_H',
       'PNG_USE_READ_MACROS',
     ],
     'include_dirs': [
       # This is implicit in GN.
       '<(DEPTH)',
       '..',
     ],
     'conditions': [
       ['pdf_enable_v8==1', {
         'defines': [
@@ skipping 61 matching lines @@
       'conditions': [
         ['pdf_enable_xfa==0', {
           'dependencies': [
             '../third_party/third_party.gyp:fx_lpng',
           ],
         }],
       ],
     },
   ],
 }