APZ hit-testing with an inactive scrollframe that is visibility:hidden (bug 1673505)

The body of this document is scrollable and is the main scrollable element. On top of that we have a hidden fixed-pos item containing another scrollframe, but this nested scrollframe is inactive. Since the fixed-pos item is hidden, the nested scrollframe is hidden too and shouldn't be the target of hit-testing. However, because it is an inactive scrollframe, code to generate the "this is an inactive scrollframe" area was marking it as hit-testable. This bug led to hit- tests being mis-targeted to the nested scrollframe's layers id instead of whatever was underneath.