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..d089512cd3f26b74056a3b39827fdbd39ccace59 100644
--- a/gyp/common_variables.gypi
+++ b/gyp/common_variables.gypi
@@ -76,10 +76,16 @@
           'skia_os%': 'android',
           'skia_chrome_utils%': 0,
           'skia_use_system_json%': 1,
+          # 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%': '0',
         }, {
           'skia_os%': '<(skia_os)',
           'skia_chrome_utils%': 1,
           'skia_use_system_json%': 0,
+          'skia_libpng_static%': '1',
         }],
         [ 'skia_os == "win"', {
           'os_posix%': 0,
@@ -129,11 +135,6 @@
       #     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