Get back on mainline libpng 1.6 now that 3bpp perf patches are merged.

DEPS

Index: DEPS
diff --git a/DEPS b/DEPS
index a9008660eca15da9b7fb044109ba6ad876dae104..137d0abd14fc7e3e198f721fe6c15c93d27d6940 100644
--- a/DEPS
+++ b/DEPS
@@ -18,8 +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@52846504da1f519cf1bb89636caf9110e8677443",
-  "third_party/externals/libpng"   : "https://github.com/mtklein/libpng.git@d0513638e7cc7d22b7dace879afea848b342fe33",
+  "third_party/externals/libpng"  : "https://skia.googlesource.com/third_party/libpng.git@5765a2224946b953ec6702bd01205af475f8c80a",
   "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",