[mediacapture-output] Where did "speaker" feature policy go? (#91)

jan-ivar has just created a new issue for https://github.com/w3c/mediacapture-output:

== Where did "speaker" feature policy go? ==
I wrote https://github.com/w3c/mediacapture-output/pull/90 before realizing `"speaker"` was  removed in https://github.com/w3c/webappsec-feature-policy/pull/360 and in Chrome. @alvestrand what happened to it?

Concerns:
 1. Without it, people may permission-escalate with `allow="microphone"` or `allow="camera"` in order to use this feature. While it's true you need those permissions today to use this feature in Chrome, this may not always be so. E.g. https://github.com/w3c/mediacapture-output/pull/86
 2. There's still a `"speaker"` [permission](https://w3c.github.io/permissions/#dom-permissionname-speaker). Without a corresponding feature-policy, it's unclear whether this feature is available in iframes by default or not (whether you have cam/mic or not).
 3. It [removes consent](https://github.com/w3c/mediacapture-output/issues/89) in iframes by default.

cc @alvestrand, @guidou, @achronop

Please view or discuss this issue at https://github.com/w3c/mediacapture-output/issues/91 using your GitHub account