Point libpng at upstream libpng 1.6 + my new perf patch.

DEPS

Index: DEPS
diff --git a/DEPS b/DEPS
index 8bc7b5fbdf7d913df919932f74ed32bf98af2caa..55134f991ebd77faf4771dea361e2a6332330232 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@52846504da1f519cf1bb89636caf9110e8677443",
+  #"third_party/externals/libpng"  : "https://skia.googlesource.com/third_party/libpng.git@52846504da1f519cf1bb89636caf9110e8677443",
+  "third_party/externals/libpng"   : "https://github.com/mtklein/libpng.git@0cf9bc11a54a18e6bc34a656b0012580aa05e70b",
   "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@1754202ed3e25b98179a4f377f0bdd605d9a157a",
@@ -50,7 +51,7 @@ deps = {
 
   # Mojo SDK for Mojo RPC experiments.
   "third_party/externals/mojo/public" : "https://chromium.googlesource.com/external/github.com/domokit/mojo_sdk@172be18ea745ab29aea132e6cb952e726dc32543",
-  
+
   # shaderc for the vulkan backend
   "third_party/externals/shaderc2" : "https://github.com/google/shaderc.git@d63c5e070fa966a33e1d352b2272bd57e1e9da00",
   "third_party/externals/shaderc2/third_party/googletest" : "https://github.com/google/googletest.git@d225acc90bc3a8c420a9bcd1f033033c1ccd7fe0",