Regression: define source browsing context for iframe and frame

1490eba, from PR #484, removed the
source browsing context definition along with "explicit
self-navigation override". This restores the definition of source
browsing context for these elements as it was before that commit.

This fixes #1131, but note that per #1130 further changes are
required here, as browsing contexts are not a good concept to use as
source.

source

@@ -27545,6 +27545,11 @@ href="?audio">audio&lt;/a> test instead.)&lt;/p></pre>
    to <var>url</var>.</p></li>
   </ol>
 
+  <p>Any <span data-x="navigate">navigation</span> required of the user agent in the <span>process
+  the <code>iframe</code> attributes</span> algorithm must use the <code>iframe</code> element's
+  <span>node document</span>'s <span>browsing context</span> as the <span>source browsing
+  context</span>.</p>
+
   <p>Furthermore, if the <span>active document</span> of the element's <span>child browsing
   context</span> before such a <span data-x="navigate">navigation</span> was not <span>completely
   loaded</span> at the time of the new <span data-x="navigate">navigation</span>, then the <span
@@ -112513,6 +112518,11 @@ if (s = prompt('What is your name?')) {
 
   </dl>
 
+  <p>Any <span data-x="navigate">navigation</span> required of the user agent in the <span>process
+  the <code>frame</code> attributes</span> algorithm must use the <code>frame</code> element's
+  <span>node document</span>'s <span>browsing context</span> as the <span>source browsing
+  context</span>.</p>
+
   <p>Furthermore, if the <span>active document</span> of the element's <span>child browsing
   context</span> before such a <span data-x="navigate">navigation</span> was not <span>completely
   loaded</span> at the time of the new <span data-x="navigate">navigation</span>, then the <span