Add test for platform encoders, turn off platform encoders by default

gyp/images.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.
 
 # GYP file for images project.
 {
   'targets': [
     {
       'target_name': 'images',
       'product_name': 'skia_images',
       'type': 'static_library',
       'standalone_static_library': 1,
       'dependencies': [
         'core.gyp:*',
         'giflib.gyp:giflib',
         'libjpeg-turbo-selector.gyp:libjpeg-turbo-selector',
         'etc1.gyp:libetc1',
         'ktx.gyp:libSkKTX',
+        'libpng.gyp:libpng',
         'libwebp.gyp:libwebp',
         'utils.gyp:utils',
       ],
       'include_dirs': [
         '../include/images',
         '../include/private',
         '../src/lazy',
         # for access to SkErrorInternals.h
         '../src/core/',
         # for access to SkImagePriv.h
@@ skipping 20 matching lines @@
         '../src/images/SkJPEGWriteUtility.cpp',
         '../src/images/SkMovie.cpp',
         '../src/images/SkGIFMovie.cpp',
 
         '../src/ports/SkImageEncoder_CG.cpp',
         '../src/ports/SkImageEncoder_WIC.cpp',
       ],
       'conditions': [
         [ 'skia_os == "win"', {
           'sources!': [
-            '../src/images/SkPNGImageEncoder.cpp',
             '../src/images/SkGIFMovie.cpp',
           ],
           'dependencies!': [
             'giflib.gyp:giflib'
           ],
           'link_settings': {
             'libraries': [
               '-lwindowscodecs.lib',
             ],
           },
         },{ #else if skia_os != win
           'sources!': [
             '../src/ports/SkImageEncoder_WIC.cpp',
           ],
         }],
         [ 'skia_os in ["mac", "ios"]', {
           'sources!': [
-            '../src/images/SkPNGImageEncoder.cpp',
             '../src/images/SkGIFMovie.cpp',
           ],
         },{ #else if skia_os != mac
           'sources!': [
             '../src/ports/SkImageEncoder_CG.cpp',
           ],
         }],
-        [ 'skia_os in ["linux", "freebsd", "openbsd", "solaris"]', {
-          'dependencies': [
-            'libpng.gyp:libpng',
-          ],
-          # end libpng stuff
-        }],
         [ 'skia_os == "android"', {
           'include_dirs': [
              '../src/utils',
           ],
           'dependencies': [
             'libpng.gyp:libpng',
           ],
           'conditions': [
             [ 'skia_android_framework == 1', {
               # The android framework disables these decoders as they are of little use to
@@ skipping 11 matching lines @@
         }],
       ],
       'direct_dependent_settings': {
         'include_dirs': [
           '../include/images',
         ],
       },
     },
   ],
 }