Define BarProp lifetime better

[annevk]

In particular we cannot tie it to browsing contexts, as those can be discarded.

const frame = self[0]
alert(frame.menubar)
frame.frameElement.remove()
alert(frame.menubar)

Firefox: obj, obj
Safari TP: obj, undefined
Chrome: obj, obj
Edge: obj, exception ("Object expected")

In Firefox and Chrome visible will return false for the case lacking a browsing context. That's probably what we should go with.

[annevk]

Per https://cs.chromium.org/chromium/src/third_party/blink/renderer/core/frame/bar_prop.cc and https://github.com/WebKit/webkit/blob/master/Source/WebCore/page/BarProp.cpp I guess there are scenarios where the individual values can differ, at least for some. Specifying that in more detail is probably best tracked separately?