Add bytes() method to Body mixin

Returns a Uint8Array as opposed to requiring users to get an ArrayBuffer and wrap it.

Tests: web-platform-tests/wpt#46198.

Fixes #1732.

fetch.bs

@@ -7162,6 +7162,7 @@ interface mixin Body {
   readonly attribute boolean bodyUsed;
   [NewObject] Promise<ArrayBuffer> arrayBuffer();
   [NewObject] Promise<Blob> blob();
+  [NewObject] Promise<Uint8Array> bytes();
   [NewObject] Promise<FormData> formData();
   [NewObject] Promise<any> json();
   [NewObject] Promise<USVString> text();
@@ -7195,6 +7196,9 @@ due course.
  <dt><code><var>requestOrResponse</var> . <a method for=Body>blob</a>()</code>
  <dd><p>Returns a promise fulfilled with <var>requestOrResponse</var>'s body as {{Blob}}.
 
+ <dt><code><var>requestOrResponse</var> . <a method for=Body>bytes</a>()</code>
+ <dd><p>Returns a promise fulfilled with <var>requestOrResponse</var>'s body as {{Uint8Array}}.
+
  <dt><code><var>requestOrResponse</var> . <a method for=Body>formData</a>()</code>
  <dd><p>Returns a promise fulfilled with <var>requestOrResponse</var>'s body as {{FormData}}.
 
@@ -7290,6 +7294,15 @@ and whose {{Blob/type}} attribute is the result of <a for=Body>get the MIME type
 <!-- This is wrong. It should really set an underlying concept to this. -->
 </div>
 
+<div algorithm>
+<p>The <dfn method for=Body><code>bytes()</code></dfn> method steps are to return the result
+of running <a for=Body>consume body</a> with <a>this</a> and the following step given a
+<a for=/>byte sequence</a> <var>bytes</var>: return the result of [=ArrayBufferView/create|creating=] a
+{{Uint8Array}} from <var>bytes</var> in <a>this</a>'s <a>relevant realm</a>.
+
+<p class="note">The above method can reject with a {{RangeError}}.
+</div>
+
 <div algorithm>
 <p>The <dfn method for=Body><code>formData()</code></dfn> method steps are to return the result of
 running <a for=Body>consume body</a> with <a>this</a> and the following steps given a