Update libpng to 1.6.22rc01

DEPS

Index: DEPS
diff --git a/DEPS b/DEPS
index 801584656762224c5d7a36c39fb4470846607888..a8a11e7d5c6e17953fd316ae52138de277ebdbdf 100644
--- a/DEPS
+++ b/DEPS
@@ -18,7 +18,8 @@ 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@5765a2224946b953ec6702bd01205af475f8c80a",
+  # skbug.com/4710: We'd prefer to mirror AOSP or Chrome.
+  "third_party/externals/libpng"  : "https://github.com/mattsarett/libpng.git@91d2a9e56ea83f7bce091b1606ae4d04ddfc547a",
   "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@96443b262250c390b0caefbf3eed8463ba35ecae",