Fix GN build after PNG support backport

samples/BUILD.gn

Index: samples/BUILD.gn
diff --git a/samples/BUILD.gn b/samples/BUILD.gn
index 37e13da639a7f6a9f62e04fe04e2a42a45dff45c..1bb15a35f514e87bc35783025f0365719a913814 100644
--- a/samples/BUILD.gn
+++ b/samples/BUILD.gn
@@ -5,19 +5,69 @@
 group("samples") {
   deps = [
     ":pdfium_test",
+    ":pdfium_diff",
+  ]
+}
+
+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",
+    ":fx_lpng",
   ]
   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",
+    ":fx_lpng",
+  ]
+  configs += [ ":pdfium_samples_config" ]
+}
+
+static_library("fx_lpng") {
+  sources = [
+    "fx_lpng/include/fx_png.h",
+    "fx_lpng/src/fx_png.c",
+    "fx_lpng/src/fx_pngerror.c",
+    "fx_lpng/src/fx_pngget.c",
+    "fx_lpng/src/fx_pngmem.c",
+    "fx_lpng/src/fx_pngpread.c",
+    "fx_lpng/src/fx_pngread.c",
+    "fx_lpng/src/fx_pngrio.c",
+    "fx_lpng/src/fx_pngrtran.c",
+    "fx_lpng/src/fx_pngrutil.c",
+    "fx_lpng/src/fx_pngset.c",
+    "fx_lpng/src/fx_pngtrans.c",
+    "fx_lpng/src/fx_pngwio.c",
+    "fx_lpng/src/fx_pngwrite.c",
+    "fx_lpng/src/fx_pngwtran.c",
+    "fx_lpng/src/fx_pngwutil.c",
+  ]
+  include_dirs = [
+    "../core/src/fxcodec/fx_zlib/include/",
+  ]
+  configs += [ ":pdfium_samples_config" ]
+}