Add ability to query UserActivation state

[dtapuska]

Proposal: https://github.com/dtapuska/useractivation

Add an API to query the user activation state and to take a snapshot of it at postMessage time.

[annevk]

See #4009.

[domenic]

Note that this is shipping in Chrome. Agenda+ to discuss if any other browsers are interested.

Personally I feel like navigator.userActivation is an easy win. The PostMessageOptions extension is less obviously clear in its utility.

[domenic]

Oh, it seems like other browsers are indeed somewhat interested, per #4009 (comment) . /cc @smaug---- and @whsieh. It's not clear to me whether that was for the full API or just the navigator.userActivation portion, but that's encouraging...

[andreubotella]

Note that since structuredClone() does nothing with user activation, and #3414 renamed the PostMessageOptions dictionary to StructuredCloneOptions, in https://chromium-review.googlesource.com/c/chromium/src/+/3245334 we decided to bring back the old dictionary name to keep the difference. See dtapuska/useractivation#15.

[annevk]

I'm a bit surprised to hear something shipped here without the PR being finalized. I had gotten the impression it was abandon-ware.