Meta: export Document's active sandboxing flag set

For Pointer Lock.

source

@@ -81220,10 +81220,10 @@ interface <dfn>BarProp</dfn> {
   set</span> are set at any particular time is determined by the <code>iframe</code> element's <code
   data-x="attr-iframe-sandbox">sandbox</code> attribute.</p>
 
-  <p>Every <code>Document</code> has an <dfn>active sandboxing flag set</dfn>, which is a
-  <span>sandboxing flag set</span>. When the <code>Document</code> is created, its <span>active
-  sandboxing flag set</span> must be empty. It is populated by the <span data-x="navigate">navigation
-  algorithm</span>.</p>
+  <p>Every <code>Document</code> has an <dfn data-dfn-for="Document" data-export="">active
+  sandboxing flag set</dfn>, which is a <span>sandboxing flag set</span>. When the
+  <code>Document</code> is created, its <span>active sandboxing flag set</span> must be empty. It is
+  populated by the <span data-x="navigate">navigation algorithm</span>.</p>
 
   <p>Every resource that is obtained by the <span data-x="navigate">navigation algorithm</span> has
   a <dfn data-export="">forced sandboxing flag set</dfn>, which is a <span>sandboxing flag