Fix XFA build after combining PDFium into Chromium binary.

BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//testing/test.gni")
 
 pdf_use_skia = false
 
 declare_args() {
   # On Android there's no system FreeType. On Windows and Mac, only a few
   # methods are used from it.
   bundle_freetype = !is_linux
 }
 
 config("pdfium_config") {
   cflags = []
   include_dirs = [ "third_party/freetype/include" ]
   defines = [
     "_FPDFSDK_LIB",
     "_NO_GDIPLUS_",  # workaround text rendering issues on Windows
     "OPJ_STATIC",
+    "PNG_PREFIX",
+    "PNGPREFIX_H",
+    "PNG_USE_READ_MACROS",
   ]
 
   if (pdf_use_skia) {
     defines += [ "_SKIA_SUPPORT_" ]
   }
 
   if (is_linux) {
     if (cpu_arch == "x64") {
       defines += [ "_FX_CPU_=_FX_X64_" ]
       cflags += [ "-fPIC" ]
@@ skipping 1550 matching lines @@
     "testing/fx_string_testhelpers.h",
   ]
   deps = [
     "//testing/gtest",
     ":pdfium"
   ]
   include_dirs = [ "." ]
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [ ":pdfium_config", "//build/config/compiler:no_chromium_code" ]
 }