Update the version of libpng used by Skia tests

DEPS

Index: DEPS
diff --git a/DEPS b/DEPS
index 683cc624fe104bb372d0195759ddc5b334c80efe..c98b3fd2dc8c10409fb4ea67cc5d13ee56caed78 100644
--- a/DEPS
+++ b/DEPS
@@ -18,7 +18,7 @@ deps = {
   "third_party/externals/nanomsg" : "https://skia.googlesource.com/third_party/nanomsg.git@0.4-beta",
   "third_party/externals/zlib"    : "https://chromium.googlesource.com/chromium/src/third_party/zlib@c4e33043fb071b6ea0a153845da625d7ed633d3d",
   # NOTE: If we update libpng, we may need to update the generated file at third_party/libpng/pnglibconf.h
-  "third_party/externals/libpng"  : "https://skia.googlesource.com/third_party/libpng.git@070a616b8275277e18ef8ee91e2ca23f7bdc67d5",
+  "third_party/externals/libpng"  : "https://skia.googlesource.com/third_party/libpng.git@047737496a77eeb97f9991919dac08ca3c149711",
   "third_party/externals/giflib"  : "https://android.googlesource.com/platform/external/giflib.git@ab10e256df4f684260ca239905b1cec727181f6c",
 
   "third_party/externals/dng_sdk" : "https://android.googlesource.com/platform/external/dng_sdk.git@6579353b8ee5d8aa1f1a96ae22798de9b41e19b8",