Stop referring to the "responsible document" concept.

This has been removed from HTML. Instead, retrieve the document's relevant
settings object (which is an environment as specified in HTML) and run the
"is a secure context" check that is defined in HTML on it.

Fixes w3c#432.

index.bs

@@ -756,7 +756,7 @@ The user agent must verify that all [=mandatory conditions=] are satisfied to en
 [=sensor type|type=] that belong to a certain [=active document=].
 
 The <dfn>mandatory conditions</dfn> are the following:
- - The given document is a [=responsible document=] of a [=secure context=].
+ - The given document's [=relevant settings object=] is a [=secure context=].
  - For each [=powerful feature/name=] from the [=sensor type=]'s associated
    [=sensor permission names=] [=ordered set|set=], the corresponding permission's
    [=permission state|state=] is "granted".