Re: [webidl] Add a [SecureContext] operator attribute (#65)

I think that this should be an exposure change, and not a variation in implementation.  That is, interfaces, attributes and methods annotated with `[SecureContext]` should not be visible to content.  Two reasons: one is that it allows us to remove attributes (e.g., `Window.crypto.subtle`) and interfaces as well as methods.

Second being that this then relies instead on feature detection rather than error handling.  In general, I think that apps treat absent features very differently to failing features.  Critically, if `getUserMedia()` were to be annotated in this fashion (as Chrome is doing), then you only learn that it is broken in this fashion when you call it.  However, calling `getUserMedia()` has side-effects that a site might try to avoid: the permission prompt.

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