Fix XFA build after combining PDFium into Chromium binary.

pdfium.gyp

 {
   'variables': {
     'pdf_use_skia%': 0,
     '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' : [
       '_FPDFSDK_LIB',
       '_NO_GDIPLUS_',  # workaround text rendering issues on Windows
       'OPJ_STATIC',
+      'PNG_PREFIX',
+      'PNGPREFIX_H',
+      'PNG_USE_READ_MACROS',
     ],
     'include_dirs': [
       'third_party/freetype/include',
     ],
     'conditions': [
       ['pdf_use_skia==1', {
         'defines': ['_SKIA_SUPPORT_'],
       }],
       ['OS=="linux"', {
         'conditions': [
@@ skipping 914 matching lines @@
         'fpdfsdk/src/fpdftext_embeddertest.cpp',
         'fpdfsdk/src/fpdfview_embeddertest.cpp',
         'testing/embedder_test.cpp',
         'testing/embedder_test.h',
         'testing/fx_string_testhelpers.cpp',
         'testing/fx_string_testhelpers.h',
       ],
     },
   ],
 }