Rename SkCodec_libpng to SkPngCodec

gyp/codec.gyp

 # Copyright 2015 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 # Copyright 2015 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # GYP file for codec project.
@@ skipping 23 matching lines @@
         '../src/core',
         '../src/utils',
       ],
       'sources': [
         '../src/codec/SkAndroidCodec.cpp',
         '../src/codec/SkBmpCodec.cpp',
         '../src/codec/SkBmpMaskCodec.cpp',
         '../src/codec/SkBmpRLECodec.cpp',
         '../src/codec/SkBmpStandardCodec.cpp',
         '../src/codec/SkCodec.cpp',
-        '../src/codec/SkCodec_libpng.cpp',
         '../src/codec/SkGifCodec.cpp',
         '../src/codec/SkIcoCodec.cpp',
         '../src/codec/SkJpegCodec.cpp',
         '../src/codec/SkJpegDecoderMgr.cpp',
         '../src/codec/SkJpegUtility_codec.cpp',
         '../src/codec/SkMaskSwizzler.cpp',
         '../src/codec/SkMasks.cpp',
+        '../src/codec/SkPngCodec.cpp',
         '../src/codec/SkSampler.cpp',
         '../src/codec/SkSampledCodec.cpp',
         '../src/codec/SkSwizzler.cpp',
         '../src/codec/SkWbmpCodec.cpp',
         '../src/codec/SkWebpAdapterCodec.cpp',
         '../src/codec/SkWebpCodec.cpp',
 
         '../src/codec/SkCodecImageGenerator.cpp',
       ],
       'direct_dependent_settings': {
         'include_dirs': [
           '../include/codec',
         ],
       },
       'defines': [
         'TURBO_HAS_SKIP',
       ],
     },
   ],
 }