browser.secureStorage: Document user-gesture requirements

[oliverdunk]

There was some discussion in the main issue for browser.secureStorage on if there should be a user-gesture requirement. We're leaning towards no here but we should add that as a non-normative note in the spec along with our thought process.

[oliverdunk]

Closing this. I am still personally interested in the API but there hasn't been any recent movement on implementation. With that in mind, it doesn't feel worthwhile updating the proposal.

From Chrome's side, we see the use case for the idea at a very high level but are worried about the number of implementations for different platforms that would be needed (using macOS / Windows Hello APIs etc.) and the long-term support costs even after this was implemented across all platforms.