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

/cc @alespergl 
atom/common/api/atom_api_native_image.cc
if (image_.HasRepresentation(gfx::Image::kImageRepSkia)) {
    isolate()->AdjustAmountOfExternalAllocatedMemory(
      - image_.ToImageSkia()->bitmap()->computeSize64());
  }
}
#if defined(OS_WIN)
HICON NativeImage::GetHICON(int size) {
  auto iter = hicons_.find(size);
  if (iter != hicons_.end())
    return iter->second.get();
  
  if (!hicon_path_.empty()) {
    hicons_[size] = std::move(ReadICOFromPath(size, hicon_path_));
    return hicons_[size].get();
  }
  
  if (image_.IsEmpty())
    return NULL;
  hicons_[size] = std::move(
      IconUtil::CreateHICONFromSkBitmap(image_.AsBitmap()));
  return hicons_[size].get();
}
#endif
v8::Local<v8::Value> NativeImage::ToPNG(mate::Arguments* args) {