Fix pdfium_test build inside Chromium.

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.
 
 {
   'target_defaults': {
     'type': 'executable',
     'dependencies': [
       '../pdfium.gyp:pdfium',
       '<(DEPTH)/v8/tools/gyp/v8.gyp:v8_libplatform',
     ],
     'include_dirs': [
       '<(DEPTH)',
       '<(DEPTH)/v8',
       '<(DEPTH)/v8/include',
     ],
+    'defines' : [
+      'PNG_PREFIX',
+      'PNGPREFIX_H',
+      'PNG_USE_READ_MACROS',
+    ],
   },
   'targets': [
     {
       'target_name': 'pdfium_test',
       'sources': [
         'pdfium_test.cc',
         'image_diff_png.cc',
       ],
     },
     {
       'target_name': 'pdfium_diff',
       'type': 'executable',
       'variables': { 'enable_wexit_time_destructors': 1, },
       'dependencies': [
         '../pdfium.gyp:fxcodec',
         '../third_party/third_party.gyp:pdfium_base',
       ],
       'include_dirs': [
         '../../',
       ],
       'sources': [
         'image_diff.cc',
         'image_diff_png.h',
         'image_diff_png.cc',
       ],
     },
   ],
 }