Honor srcdoc document referrer policies when set

If a srcdoc document contains a meta element with a referrer policy, we
should use that as the document's referrer policy instead of walking up
the tree to find the first non-srcdoc document's referrer policy.

(As discussed in whatwg#1559 (comment))

source

@@ -78137,9 +78137,11 @@ console.assert(iframeWindow.frameElement === null);
        <li><p>Let <var>document</var> be the <code>Document</code> with which <var>window</var> is
        currently associated.</p></li>
 
-       <li><p>While <var>document</var> is <span>an <code>iframe</code> <code
-       data-x="attr-iframe-srcdoc">srcdoc</code> document</span>, set <var>document</var> to
-       <var>document</var>'s <span data-x="concept-document-bc">browsing context</span>'s
+       <li><p>While <var>document</var> is <span>an <code>iframe</code>
+       <code data-x="attr-iframe-srcdoc">srcdoc</code> document</span> and <var>document</var>'s
+       <span data-x="concept-document-referrer-policy">referrer policy</span> is not the empty
+       string, set <var>document</var> to <var>document</var>'s
+       <span data-x="concept-document-bc">browsing context</span>'s
        <span>browsing context container</span>'s <span>node document</span>.</p></li>
 
        <li><p>Return <var>document</var>'s <span data-x="concept-document-referrer-policy">referrer