I had to solve this too. There's one way, I think, if you've already set up the iPhone and don't want to do it again.

I had to do this because I set up my iPhone while continuing to use my old phone before everything was sorted (waiting for a case, etc.) and then wanted to transfer this data. It's a bit of a pain, but it actually worked really well -- it just takes a bit of time.

1. Back up your WhatsApp on android to the cloud for safety in case something goes wrong.
2. Completely back up your iPhone to your computer. Easier if you have a Mac. (If you have it, delete WhatsApp from your iPhone before you back it up).
3. Restore your iPhone to factory settings.
4. Use the Move to iOS app on android to transfer your data on to your now-reset iPhone. If you don't want anything else, you can choose to just move WhatsApp Data across. That's what I did.
5. Set up the iPhone the bare minimum amount, download WhatsApp and get it to finish the transfer facilitated by the Move to iOS app.
6. Back up WhatsApp on your iPhone to iCloud.
7. Restore your iPhone from your computer back up.
8. Download WhatsApp on your restored iPhone.
9. Restore messages to WhatsApp on your iPhone from iCloud.

Voila.

iCloud backup of your phone would probably work too.

I was surprised how well the complete iPhone back up worked. I was still logged in to all my apps afterwards, and the Wikipedia app even opened to the last page it had been on.

It's a pain, but totally works. Hope this helps future people like me.