Add SkCodec, including PNG implementation.

gyp/common_variables.gypi

Index: gyp/common_variables.gypi
diff --git a/gyp/common_variables.gypi b/gyp/common_variables.gypi
index 97501871a9a0607884aadd204199c57fff678135..dd2c084ff54bb179f17e43ad8c71299ecbea536d 100644
--- a/gyp/common_variables.gypi
+++ b/gyp/common_variables.gypi
@@ -121,6 +121,16 @@
             'skia_freetype_static%': '0',
           }
         ],
+        [ 'skia_os in ["mac", "ios", "win"]', {
+            # skia_libpng_static - instead of linking libpng with '-lpng' and
+            #     including the headers from '/usr/include/png.h', compile and
+            #     statically link the version of libpng in
+            #     third_party/externals/libpng.
+            'skia_libpng_static%': '1',
+          }, {
+            'skia_libpng_static%': '0',
+          }
+        ],
       ],
 
       # skia_giflib_static - on OS variants that normally would link giflib
@@ -129,11 +139,6 @@
       #     giflib in third_party/externals/giflib.
       'skia_giflib_static%': '0',
 
-      # skia_libpng_static - on OS variants that normally would link libpng
-      #     with '-lpng' and include the headers from '/usr/include/png.h',
-      #     don't do that; instead compile and staticlly link the version of
-      #     libpng in third_party/externals/libpng.
-      'skia_libpng_static%': '0',
 
       # skia_no_fontconfig - On POSIX systems that would normally use the
       #     SkFontHost_fontconfig interface; use the SkFontHost_linux