Change document's fully active condition

A document without a browsing context shouldn't be fully active.
See https://bugzilla.mozilla.org/show_bug.cgi?id=1218456 for context.

PR: #462

source

@@ -77489,11 +77489,12 @@ dictionary <dfn>DragEventInit</dfn> : <span>MouseEventInit</span> {
 
   </ol>
 
-  <p>A <code>Document</code> is said to be <dfn>fully active</dfn> when it is the <span>active
-  document</span> of its <span>browsing context</span>, and either its browsing context is a
-  <span>top-level browsing context</span>, or it has a <span>parent browsing context</span> and the
-  <code>Document</code> <span data-x="browsing context nested through">through which</span> it is
-  <span data-x="nested browsing context">nested</span> is itself <span>fully active</span>.</p>
+  <p>A <code>Document</code> is said to be <dfn>fully active</dfn> when it has a <span
+  data-x="browsing context">browsing context</span> and it is the <span>active document</span> of
+  that <span>browsing context</span>, and either its browsing context is a <span>top-level browsing
+  context</span>, or it has a <span>parent browsing context</span> and the <code>Document</code>
+  <span data-x="browsing context nested through">through which</span> it is <span data-x="nested
+  browsing context">nested</span> is itself <span>fully active</span>.</p>
 
   <p>Because they are nested through an element, <span data-x="child browsing context">child browsing
   contexts</span> are always tied to a specific <code>Document</code> in their <span>parent browsing