Add SkCodec, including PNG implementation.

DEPS

Index: DEPS
diff --git a/DEPS b/DEPS
index 3c16bd9b08d680275f132dc1659f5b5bee17b7ca..15917a967bbe2b5ca0993349fa3b980d7ec244dd 100644
--- a/DEPS
+++ b/DEPS
@@ -9,15 +9,17 @@ deps = {
   #   - both Android and ChromeOS pull the same giflib;
   #   - 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@c415283b2bcd786e1a8c55c19ef3511eb2b3928c",
-  "third_party/externals/freetype" : "https://skia.googlesource.com/third_party/freetype2.git@VER-2-5-0-1",
-  "third_party/externals/gyp" : "https://chromium.googlesource.com/external/gyp.git@dd831fd86e7a254c696f53944333562466e453ad",
+  "third_party/externals/angle2"  : "https://chromium.googlesource.com/angle/angle.git@c415283b2bcd786e1a8c55c19ef3511eb2b3928c",
+  "third_party/externals/freetype": "https://skia.googlesource.com/third_party/freetype2.git@VER-2-5-0-1",
+  "third_party/externals/gyp"     : "https://chromium.googlesource.com/external/gyp.git@dd831fd86e7a254c696f53944333562466e453ad",
   "third_party/externals/harfbuzz": "https://skia.googlesource.com/third_party/harfbuzz.git@0.9.35",
   "third_party/externals/jsoncpp" : "https://chromium.googlesource.com/external/jsoncpp/jsoncpp.git@1afff032c83e26ddf7f2776e8b43de5ad666c1fa",
   "third_party/externals/libjpeg" : "https://chromium.googlesource.com/chromium/deps/libjpeg_turbo.git@82ce8a6d4ebe12a177c0c3597192f2b4f09e81c3",
   "third_party/externals/libwebp" : "https://chromium.googlesource.com/webm/libwebp.git@3fe91635df8734b23f3c1b9d1f0c4fa8cfaf4e39",
   "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@4ba7cdd0e7bf49d671645264f839838fc56e1492",
+  # NOTE: If we update libpng, we may need to update the generated file at third_party/libpng/pnglibconf.h
+  "third_party/externals/libpng"  : "git://git.code.sf.net/p/libpng/code@070a616b8275277e18ef8ee91e2ca23f7bdc67d5",
 
   "platform_tools/android/third_party/externals/expat" : "https://android.googlesource.com/platform/external/expat.git@android-4.2.2_r1.2",
   "platform_tools/android/third_party/externals/gif" : "https://android.googlesource.com/platform/external/giflib.git@android-4.2.2_r1.2",