Update libpng to 1.6.22beta

DEPS

Index: DEPS
diff --git a/DEPS b/DEPS
index c98b3fd2dc8c10409fb4ea67cc5d13ee56caed78..ebcab6fcff10e1353cea206e38f70a8c4a7091eb 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@047737496a77eeb97f9991919dac08ca3c149711",
+  # FIXME: Once the 1.6.22 release is available, we should return to mirroring the libpng at skia.googlesource.com

[scroggo, 2016/02/19 17:21:23]

The mirror actually has this commit, so no need to switch to github. (Yay for a
working mirror!)

[msarett, 2016/02/19 18:36:18]

Wow you're right!  Woohoo!

+  "third_party/externals/libpng"  : "https://github.googlesource.com/glennrp/libpng.git@52846504da1f519cf1bb89636caf9110e8677443",
   "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",