Allow platforms to save user input for future autofill

[LongCatIsLooong]

Continuation of #13015.

Android and many web browsers save user input for future autofill, when a form is finalized.

Android Documentation: https://developer.android.com/guide/topics/text/autofill-optimize#ensure

[SF-Simon]

Thank you for your efforts.
Will it be available in the first quarter of 2020?

I don't seem to see anyone talking about Automatic Strong Passwords.
Is this the same problem? This problem will be solved together, right?

about iOS Automatic Strong Passwords:
https://developer.apple.com/videos/play/wwdc2018/204

We are all looking forward to it.

[ferhatb]

Note: Required for web implementation of autofill.

[nturgut]

Hi @LongCatIsLooong ,

Did you had time to have a look at this issue? Please let me know if there are any parts I can help.

[goderbauer]

Assigning to @LongCatIsLooong since you have a PR out for this.