Update libpng to 1.6.22rc01

DEPS

 use_relative_paths = True
 
 # Dependencies on outside packages.
 #
 deps = {
   "common": "https://skia.googlesource.com/common.git@c282fe0b6e392b14f88d647cbd86e1a3ef5498e0",
 
   # There is some duplication here that might be worth cleaning up:
   #   - can use use our existing t_p/e/libjpeg instead of pulling it for Android?
 
   "third_party/externals/angle2"  : "https://chromium.googlesource.com/angle/angle.git@8182049470aa3ba0c9f7c54ef965bf1877df08cb",
   "third_party/externals/cmake"   : "https://cmake.googlesource.com/cmake.git@v3.3.1",
   "third_party/externals/freetype": "https://skia.googlesource.com/third_party/freetype2.git@6a19a7d332c5446542196e5aeda0ede109ef097b",
   "third_party/externals/gyp"     : "https://chromium.googlesource.com/external/gyp.git@87ac4d0e63fc7dd8152a350327fea8dcf031bf56",
   "third_party/externals/harfbuzz": "https://skia.googlesource.com/third_party/harfbuzz.git@1.2.7",
   "third_party/externals/jsoncpp" : "https://chromium.googlesource.com/external/github.com/open-source-parsers/jsoncpp.git@1.0.0",
   "third_party/externals/libwebp" : "https://chromium.googlesource.com/webm/libwebp.git@v0.5.0",
   "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",
   "third_party/externals/piex"    : "https://android.googlesource.com/platform/external/piex.git@be908191d0a6883a95333bdc0bca749c9b830969",
 
   "third_party/externals/libjpeg-turbo"             : "https://skia.googlesource.com/third_party/libjpeg-turbo.git@debddedc75850bcdeb8a57258572f48b802a4bb3",
   # libjpeg-turbo depends on yasm to compile .asm files
   "third_party/externals/yasm/source/patched-yasm/" : "https://chromium.googlesource.com/chromium/deps/yasm/patched-yasm.git@4671120cd8558ce62ee8672ebf3eb6f5216f909b",
   "third_party/externals/yasm/binaries"             : "https://chromium.googlesource.com/chromium/deps/yasm/binaries.git@52f9b3f4b0aa06da24ef8b123058bb61ee468881",
 
@@ skipping 26 matching lines @@
     # xSAN bots build Clang from scratch.
     "third_party/externals/llvm": "https://llvm.googlesource.com/llvm@release_38",
     "third_party/externals/llvm/tools/clang": "https://llvm.googlesource.com/clang@release_38",
     "third_party/externals/llvm/projects/compiler-rt": "https://llvm.googlesource.com/compiler-rt@release_38",
     "third_party/externals/llvm/projects/libcxx": "https://llvm.googlesource.com/libcxx@release_38",
     "third_party/externals/llvm/projects/libcxxabi": "https://llvm.googlesource.com/libcxxabi@release_38",
   }
 }
 
 recursedeps = [ "common" ]