Associate service worker timing info with a response

Helps with w3c/ServiceWorker#1575 and w3c/navigation-timing#143.

Author: noamr

fetch.bs

@@ -1968,6 +1968,10 @@ this is also tracked internally using the request's <a for=request>timing allow
 <dfn for=response id=concept-response-timing-info>timing info</dfn> (null or a
 <a for=/>fetch timing info</a>), which is initially null.
 
+<p>A <a for=/>response</a> has an associated
+<dfn export for=response>service worker timing info</dfn> (null or a
+<a for=/>service worker timing info</a>), which is initially null.
+
 <hr>
 
 <p>A <a for=/>response</a> whose