They were disabled during the Chromium 61 upgrade. Fix them and enable.

/cc @alespergl 
atom/common/api/atom_api_native_image.cc
v8::Local<v8::Context> context, void* priv) {
  mate::Dictionary dict(context->GetIsolate(), exports);
  dict.SetMethod("createEmpty", &atom::api::NativeImage::CreateEmpty);
  dict.SetMethod("createFromPath", &atom::api::NativeImage::CreateFromPath);
  dict.SetMethod("createFromBuffer", &atom::api::NativeImage::CreateFromBuffer);
  dict.SetMethod("createFromDataURL",
                 &atom::api::NativeImage::CreateFromDataURL);
  dict.SetMethod("createFromNamedImage",
                 &atom::api::NativeImage::CreateFromNamedImage);
}
}  
NODE_MODULE_CONTEXT_AWARE_BUILTIN(atom_common_native_image, Initialize)