Fix "active frame element" definition

Closes #9136 by adding a non-null browsing context check. Also updates to use "in a document tree" instead of "in a document".

source

@@ -127242,8 +127242,9 @@ interface <dfn interface>HTMLFrameSetElement</dfn> : <span>HTMLElement</span> {
 
   <!-- START of section that's very similar to <iframe> -->
 
-  <p>A <code>frame</code> element is said to be an <dfn>active <code>frame</code> element</dfn> when
-  it is <span>in a document</span>.</p>
+  <p>A <code>frame</code> element is said to be an <dfn>active <code>frame</code> element</dfn>
+  when it is <span>in a document tree</span> and its <span>node document</span>'s <span
+  data-x="concept-document-bc">browsing context</span> is non-null.</p>
   <!-- even if the parent isn't a <frameset>. In fact, even if its parent isn't being rendered at
        all: https://software.hixie.ch/utilities/js/live-dom-viewer/?saved=2721 -->
 
@@ -134528,6 +134529,7 @@ INSERT INTERFACES HERE
   Lucas Gadani,
   &#x0141;ukasz Pilorz,
   Luke Kenneth Casson Leighton,
+  Luke Wilde,
   Maciej Stachowiak,
   Magne Andersson,
   Magnus Kristiansen<!-- Dashiva -->,