Fix pdfium_test build inside Chromium.

samples/BUILD.gn

 # 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.
 
 group("samples") {
   deps = [
     ":pdfium_test",
   ]
 }
 
+config("pdfium_samples_config") {
+  defines = [
+    "PNG_PREFIX",
+    "PNGPREFIX_H",
+    "PNG_USE_READ_MACROS",
+  ]
+}
+
 executable("pdfium_test") {
   sources = [
     "image_diff_png.cc",
     "pdfium_test.cc",
   ]
   deps = [
     "//third_party/pdfium",
     "//v8:v8_libplatform",
   ]
   include_dirs = [
     "//v8",
     "//v8/include",
   ]
+  configs += [ ":pdfium_samples_config" ]
 }
 
 executable("pdfium_diff") {
   sources = [
     "image_diff.cc",
     "image_diff_png.cc",
     "image_diff_png.h",
   ]
   deps = [
     "//third_party/pdfium",
   ]
+  configs += [ ":pdfium_samples_config" ]
 }