Changed uses of JSON string to USVString

[selfissued]

Fixes #417

---

Preview | Diff

[rlin1]

Is it this USVString: http://docs.w3cub.com/dom/usvstring/

[jcjones]

@bzbarsky - can you confirm for us that USVString is what we should be using to pass user-supplied textual strings around?

[bzbarsky]

Does "user" mean "actual person sitting at a keyboard"?

If so, then I'd think USVString is a reasonable option. @annevk may have better guidance somewhere, apart from whatever is at https://heycam.github.io/webidl/#idl-USVString

If you plan to pass the string to someone who expects "Unicode text", then USVString is a good idea.

[annevk]

I think that's it. We also have https://infra.spec.whatwg.org/#strings but it doesn't mention IDL types (at some point IDL will be rewritten in terms of Infra at which point it'll reference all of that).