Rename png_ functions conflicting with chrome.

testing/libfuzzer/fuzzers.gyp

 # Copyright 2016 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': [
           'PDF_ENABLE_V8',
@@ skipping 40 matching lines @@
     }],
   ],
   # Empty target so that nonxfa builds work.
   'targets': [
     {
       'target_name': 'empty_target',
       'type': 'none',
     }
   ],
 }