[storage] Allow script to request permission only if there would be no prompt (#9)

We've heard from sites that they don't want to prompt the user for this capability and would only request permission if the browser would autogrant it. We are planning on autogranting if the origin has a high "engagement score" for this user. But we want the origin to have to request the permission, we aren't going to opt the origin in to persistence if it doesn't ask for it.

So we'd like a way for an origin to request permission only if the user wouldn't be prompted. One way would be to add an optional parameter to requestPersistent(): `requestPersistent{"noPrompt": true})`. Another would be to add a fourth permission, `"wouldBeAutoGranted"` or something, that would let the site know that `requestPersistent()` would change the default box's mode to "persistent" without prompting. I'm sure there are other strong possibilities too.

/cc @inexorabletash

---
Reply to this email directly or view it on GitHub:
https://github.com/whatwg/storage/issues/9