Check chunk type of body's stream when passed to a service worker

This change addresses the inconsistency discussed in #267 that a chunk type in a stream body on uploading to the network is limited to Uint8Array, but passing it to a service worker is not restricted. This change limits the latter as well.

Tests: web-platform-tests/wpt#28203.

fetch.bs

@@ -4155,6 +4155,37 @@ these steps:
    <li><p>Let <var>requestForServiceWorker</var> be a <a for=request>clone</a> of
    <var>request</var>.
 
+   <li>
+    <p>If <var>requestForServiceWorker</var>'s <a for=/>body</a> is non-null, then:
+
+    <ol>
+     <li>
+      <p>Let <var>transformAlgorithm</var> given <var>chunk</var> be these steps:
+
+      <ol>
+       <li><p>If the ongoing fetch is <a for=fetch>terminated</a>, then abort these steps.
+
+       <li><p>If <var>chunk</var> is not a {{Uint8Array}} object, then
+       <a lt=terminated for=fetch>terminate</a> the ongoing fetch.
+
+       <li><p>Otherwise, <a for=ReadableStream>enqueue</a> <var>chunk</var>. The user agent may
+       split the chunk into <a>implementation-defined</a> practical sizes and
+       <a for=ReadableStream>enqueue</a> each of them. The user agent also may concatenate the
+       chunks into an <a>implementation-defined</a> practical size and
+       <a for=ReadableStream>enqueue</a> it.
+      </ol>
+
+     <li><p>Let <var>transformStream</var> be the result of <a for=TransformStream>setting up</a> a
+     {{TransformStream}} with <var>transformAlgorithm</var>.
+
+     <li><p>Let <var>transformedStream</var> be the result of <var>requestForServiceWorker</var>'s
+     <a for=/>body</a>'s <a for=body>stream</a> <a for=ReadableStream>piped through</a>
+     <var>transformStream</var>.
+
+     <li><p>Set <var>requestForServiceWorker</var>'s <a for=/>body</a>'s <a for=body>stream</a> to
+     <var>transformedStream</var>.
+    </ol>
+
    <li><p>Let <var>serviceWorkerStartTime</var> be the <a for=/>coarsened shared current time</a>
    given <var>fetchParams</var>'s <a for="fetch params">cross-origin isolated capability</a>.
 
@@ -7828,6 +7859,7 @@ Xabier Rodríguez,
 Yehuda Katz,
 Yoav Weiss,
 Youenn Fablet<!-- youennf; GitHub -->,
+Yoichi Osato,
 平野裕 (Yutaka Hirano), and
 Zhenbin Xu
 for being awesome.