Make FetchEvent be cancelable. Fixes #741.

spec/service_worker/index.html

@@ -3006,7 +3006,7 @@ <h1>Handle Fetch</h1>
       <li>Invoke <a href="#run-service-worker-algorithm">Run Service Worker</a> algorithm with <var>activeWorker</var> as the arguement.</li>
       <li><a href="https://html.spec.whatwg.org/multipage/webappapis.html#queue-a-task">Queue a task</a> <var>task</var> to run the following substeps:
         <ol>
-          <li>Create a <a href="https://html.spec.whatwg.org/#concept-events-trusted">trusted</a> event <var>e</var> that uses the <code><a href="#fetch-event-interface">FetchEvent</a></code> interface, with the event type <code><a href="#service-worker-global-scope-fetch-event">fetch</a></code>, which does not bubble, is not cancelable, and has no default action.</li>
+          <li>Create a <a href="https://html.spec.whatwg.org/#concept-events-trusted">trusted</a> event <var>e</var> that uses the <code><a href="#fetch-event-interface">FetchEvent</a></code> interface, with the event type <code><a href="#service-worker-global-scope-fetch-event">fetch</a></code>, which does not bubble and has no default action.</li>
           <li>Let the <a href="#fetch-event-request-attribute">request</a> attribute of <var>e</var> be initialized to <var>r</var>.</li>
           <li>Let <var>c</var> be null.</li>
           <li>If <var>request</var>'s <a href="https://fetch.spec.whatwg.org/#concept-request-client">client</a> is a <a href="#dfn-window-client">window client</a>, set <var>c</var> to the result of running <a href="#capture-windowclient-algorithm">Capture Window Client</a> algorithm, or its <a href="#dfn-processing-equivalence">equivalent</a>, with <var>request</var>'s <a href="https://fetch.spec.whatwg.org/#concept-request-client">client</a>'s <a href="https://html.spec.whatwg.org/multipage/webappapis.html#responsible-browsing-context">responsible browsing context</a> as the argument.</li>