XFA: Upgrade libpng to 1.6.20.

third_party/BUILD.gn

Index: third_party/BUILD.gn
diff --git a/third_party/BUILD.gn b/third_party/BUILD.gn
index 5f35432236ee8ee39e176a1e5d85014c547cc545..d8d1dd227072d7b140590a48de8871f287953f23 100644
--- a/third_party/BUILD.gn
+++ b/third_party/BUILD.gn
@@ -278,22 +278,28 @@ source_set("fx_lpng") {
     "//third_party/pdfium:pdfium_config",
   ]
   sources = [
-    "lpng_v163/png.h",
-    "lpng_v163/png.c",
-    "lpng_v163/pngerror.c",
-    "lpng_v163/pngget.c",
-    "lpng_v163/pngmem.c",
-    "lpng_v163/pngpread.c",
-    "lpng_v163/pngread.c",
-    "lpng_v163/pngrio.c",
-    "lpng_v163/pngrtran.c",
-    "lpng_v163/pngrutil.c",
-    "lpng_v163/pngset.c",
-    "lpng_v163/pngtrans.c",
-    "lpng_v163/pngwio.c",
-    "lpng_v163/pngwrite.c",
-    "lpng_v163/pngwtran.c",
-    "lpng_v163/pngwutil.c",
+    "libpng/png.c",
+    "libpng/png.h",
+    "libpng/pngconf.h",
+    "libpng/pngdebug.h",
+    "libpng/pngerror.c",
+    "libpng/pngget.c",
+    "libpng/pnginfo.h",
+    "libpng/pnglibconf.h",
+    "libpng/pngmem.c",
+    "libpng/pngpread.c",
+    "libpng/pngpriv.h",
+    "libpng/pngread.c",
+    "libpng/pngrio.c",
+    "libpng/pngrtran.c",
+    "libpng/pngrutil.c",
+    "libpng/pngset.c",
+    "libpng/pngstruct.h",
+    "libpng/pngtrans.c",
+    "libpng/pngwio.c",
+    "libpng/pngwrite.c",
+    "libpng/pngwtran.c",
+    "libpng/pngwutil.c",
   ]
 }