CREDENTIAL: Skip service workers for chooser images.

Closes #246.

Author: mikewest

specs/credentialmanagement/index.src.html

@@ -1987,7 +1987,10 @@ <h3 id="privacy-chooser-leakage">Chooser Leakage</h3>
   cache them for the lifetime of the {{Credential}}.
 
   Further, these images MUST be fetched with the <code>credentials</code> mode
-  set to "<code>omit</code>".
+  set to "<code>omit</code>", the <code>skip-service-worker flag</code> set, the
+  <code>client</code> set to <code>null</code>, the <code>initiator</code> set
+  to the empty string, and the <code>destination</code> set to
+  <code>subresource</code>.
 
   <h3 id="locally-stored-data">Locally Stored Data</h3>