Based on https://github.com/octo-code/webp-unity3d
Windows build of libwebp 0.5.2 were taken from here:
- https://s3.amazonaws.com/resizer-dynamic-downloads/webp/0.5.2/x86_64/libwebp.dll
- https://s3.amazonaws.com/resizer-dynamic-downloads/webp/0.5.2/x86/libwebp.dll
To include this into your project, you can use npm method of unity package manament described here. After installing this package, there's one extra step: Adding --unsafe to Assets/smcr.rsp.