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

/cc @alespergl 
atom/common/api/atom_api_native_image.cc
float scale_factor = GetScaleFactorFromOptions(args);
  const SkBitmap bitmap =
      image_.AsImageSkia().GetRepresentation(scale_factor).sk_bitmap();
  SkPixelRef* ref = bitmap.pixelRef();
  if (!ref)
    return node::Buffer::New(args->isolate(), 0).ToLocalChecked();
  return node::Buffer::Copy(args->isolate(),
                            reinterpret_cast<const char*>(ref->pixels()),
                            bitmap.getSafeSize()).ToLocalChecked();
}
v8::Local<v8::Value> NativeImage::ToJPEG(v8::Isolate* isolate, int quality) {
  std::vector<unsigned char> output;
  gfx::JPEG1xEncodedDataFromImage(image_, quality, &output);
  if (output.empty())
    return node::Buffer::New(isolate, 0).ToLocalChecked();
  return node::Buffer::Copy(
      isolate,
      reinterpret_cast<const char*>(&output.front()),
      output.size()).ToLocalChecked();
}
std::string NativeImage::ToDataURL(mate::Arguments* args) {