title: How to set cookies on a response in a ServiceWorker
body: <p>In a fetch handler triggered by a page navigation, I tried to do this:</p>
  
  <pre><code>return event.respondWith(new Response('Hello!', {
    headers: {
      "Set-Cookie": "TestCookie=foo; path=/; Max-Age=60;"
      "TestHeader": "foo"
    }
  }));
  </code></pre>
  
  <p>Then I loaded any URL in the browser, and got the "Hello!" body.  In Chrome devtools, I see the <code>TestHeader</code> set in the network panel. But the cookie is not showing up in the network panel, nor in the Application > Cookies viewer.  <code>document.cookie</code> also fails to produce it.</p>
  
  <p>The request is initiated by a page navigation, so there's no opportunity to set <code>credentials: "include"</code> on the fetch from the browser tab.</p>
  
  <p>Is it possible to add a cookie to a response in the ServiceWorker?  If not, is it possible to write cookies in any other way?</p>