title: Identify request's page origin in ServiceWorker
body: <p>In a <code>ServiceWorker</code>'s <code>onfetch</code> handler I want to be able to identify what page a <code>Request</code> is coming from.  For example, if I have 2 pages that both request <code>/resource.js</code>, and this request is passed to the service worker (the <code>onfetch</code> handler is called with a <code>FetchEvent</code>), how do I tell if it is one instance of a page requesting <code>/resource.js</code> twice, or 2 different pages requesting <code>/resource.js</code>?</p>
  
  <p>Is this possible?</p>