Tee request body on service worker interception

Also stop transmitting request's body in this scenario.

Service worker PR: w3c/ServiceWorker#1563.

Tests: web-platform-tests/wpt#27325.

Fixes w3c/ServiceWorker#1560 and fixes #572.

fetch.bs

@@ -3804,14 +3804,18 @@ these steps:
   <p>If <var>request</var>'s <a>service-workers mode</a> is "<code>all</code>", then:
 
   <ol>
+   <li><p>Let <var>requestForServiceWorker</var> be a <a for=request>clone</a> of
+   <var>request</var>.
+
    <li><p>Set <var>response</var> to the result of invoking <a for=/>handle fetch</a> for
-   <var>request</var>. [[!HTML]] [[!SW]]
+   <var>requestForServiceWorker</var>. [[!HTML]] [[!SW]]
 
    <li>
     <p>If <var>response</var> is not null, then:
 
     <ol>
-     <li><p><a>Transmit request body</a> given <var>fetchParams</var>.
+     <li>If <var>request</var>'s <a for=request>body</a> is non-null, then
+     <a for=ReadableStream>cancel</a> <var>request</var>'s <a for=request>body</a> with undefined.
 
      <li><p>Set <var>actualResponse</var> to <var>response</var>, if <var>response</var> is not a
      <a>filtered response</a>, and to <var>response</var>'s