Make about:srcdoc traversals use policy container from history

This is the current behavior of Chrome, and is that of Firefox for CSP.

Part of #6809.

Author: ArthurSonzogni

source

@@ -86508,8 +86508,8 @@ interface <dfn interface>BarProp</dfn> {
    must have been deleted, too. We might want to store the policy container alongside the blob URL
    entry directly (https://github.com/w3c/FileAPI/issues/142), and then reconsider this. -->
 
-   <li><p>If <var>url</var> is <code>about:srcdoc</code>, then return false.</p></li>
-
+   <!-- See about:srcdoc discussion:
+   https://github.com/whatwg/html/issues/6809#issuecomment-1170924595 -->
    <li><p>If <var>url</var> <span data-x="is-local">is local</span>, then return true.</p></li>
 
    <li><p>Return false.</p></li>