Update integration with Mixed Content

Tests: web-platform-tests/wpt#37633.

Author: carlosjoan91

fetch.bs

@@ -4338,6 +4338,8 @@ steps:
 
  <li><p><a>Upgrade <var>request</var> to a potentially trustworthy URL, if appropriate</a>.
 
+ <li><p><a>Upgrade a mixed content <var>request</var> to a potentially trustworthy URL, if appropriate</a>.
+
  <li><p>If <a lt="block bad port">should <var>request</var> be blocked due to a bad port</a>,
  <a lt="should fetching request be blocked as mixed content?">should fetching <var>request</var> be blocked as mixed content</a>, or
  <a lt="should request be blocked by Content Security Policy?">should <var>request</var> be blocked by Content Security Policy</a>