[mediacapture-main] Pull Request: Fix nonsense s/active document/responsible document/ in devicechange (editorial)

jan-ivar has just submitted a new pull request for https://github.com/w3c/mediacapture-main:

== Fix nonsense s/active document/responsible document/ in devicechange (editorial) ==
This is an editorial fix.

The intent here was always to exclude background tabs, yet this check did not accomplish that: Checking whether a browsing context's *"active document* is *"fully active"* is circular and almost always true, except for some iframes.

See https://github.com/w3c/mediacapture-main/pull/567