Remove browsing context check from WakeLock.request()

Since whatwg/html#6315, the HTML spec suggests other specifications use
"navigable" and associated concepts (along with Document) rather than
"browsing context" in most cases.

In this specific case, however, we can simply remove the step that checks if
`document`'s browsing context is null -- there is no case in which a
document is fully active _and_ has a null browsing context, as confirmed by
whatwg/html#9509.

Fixes #362.

index.html

@@ -327,10 +327,6 @@ <h3>
           |document|, return [=a promise rejected with=] a
           {{"NotAllowedError"}} {{DOMException}}.
           </li>
-          <li>If the |document|'s [=Document/browsing context=] is `null`,
-          return [=a promise rejected with=] {{"NotAllowedError"}}
-          {{DOMException}}.
-          </li>
           <li data-tests="wakelock-active-document.https.window.html">If
           |document| is not [=Document/fully active=], return [=a promise
           rejected with=] with a {{"NotAllowedError"}} {{DOMException}}.