Update skia to test with libpng to 1.6.10

DEPS

Index: DEPS
diff --git a/DEPS b/DEPS
index 50707d450a0777ab24b5f56abbf7987685fcc895..6fa107a0fe12e1839fa2fea12821c78696968e8a 100644
--- a/DEPS
+++ b/DEPS
@@ -27,7 +27,7 @@ deps = {
 
   "platform_tools/android/third_party/externals/expat" : "https://android.googlesource.com/platform/external/expat.git@android-5.1.0_r3",
   "platform_tools/android/third_party/externals/jpeg" : "https://android.googlesource.com/platform/external/jpeg.git@android-5.1.0_r3",
-  "platform_tools/android/third_party/externals/png" : "https://android.googlesource.com/platform/external/libpng.git@android-4.2.2_r1.2",
+  "platform_tools/android/third_party/externals/png" : "https://android.googlesource.com/platform/external/libpng.git@android-5.1.1_r13",
 
   "platform_tools/chromeos/toolchain/src/third_party/chromite": "https://chromium.googlesource.com/chromiumos/chromite.git@d6a4c7e0ee4d53ddc5238dbddfc0417796a70e54",
   "platform_tools/chromeos/toolchain/src/third_party/pyelftools": "https://chromium.googlesource.com/chromiumos/third_party/pyelftools.git@bdc1d380acd88d4bfaf47265008091483b0d614e",