Fix ChromeOS build broken in f84722e4777

gyp/images.gyp

 # GYP file for images project.
 {
   'targets': [
     {
       'target_name': 'images',
       'product_name': 'skia_images',
       'type': 'static_library',
       'standalone_static_library': 1,
       'dependencies': [
         'core.gyp:*',
@@ skipping 144 matching lines @@
           ],
         },{ #else if skia_os != android
           'sources!': [
             '../src/images/SkImageRef_ashmem.h',
             '../src/images/SkImageRef_ashmem.cpp',
           ],
         }],
         [ 'skia_os == "chromeos"', {
           'dependencies': [
              'chromeos_deps.gyp:gif',
+             'libpng.gyp:libpng',
           ],
         }],
         [ 'skia_os == "ios"', {
            'include_dirs': [
              '../include/utils/mac',
            ],
         }],
       ],
       'direct_dependent_settings': {
         'include_dirs': [
           '../include/images',
         ],
       },
     },
   ],
 }