Support redirectUrl at onHeadersReceived in WebRequest / DWR API

chrome/common/extensions/docs/templates/intros/webRequest.html

 <h2 id="manifest">Manifest</h2>
 <p>You must declare the "webRequest" permission in the <a
   href="manifest.html">extension manifest</a> to use the web request
 API, along with <a href="declare_permissions.html">host permissions</a>
 for any hosts whose network requests you want to access. If you want to
 use the web request API in a blocking fashion, you need to request
 the "webRequestBlocking" permission in addition.
 For example:</p>
 <pre data-filename="manifest.json">
 {
@@ skipping 40 matching lines @@
   <dt><code>onSendHeaders</code></dt>
   <dd>Fires after all extensions have had a chance to modify the request
   headers, and presents the final <a href="#life_cycle_footnote">(*)</a>
   version. The event is triggered before the headers are sent to the network.
   This event is informational and handled asynchronously. It does not allow
   modifying or cancelling the request.</dd>
   <dt><code>onHeadersReceived</code> (optionally synchronous)</dt>
   <dd>Fires each time that an HTTP(S) response header is received. Due
   to redirects and authentication requests this can happen multiple times per
   request. This event is intended to allow extensions to add, modify, and delete
-  response headers, such as incoming Set-Cookie headers.</dd>
+  response headers, such as incoming Set-Cookie headers. It also allows you to
+  redirect the request.</dd>
   <dt><code>onAuthRequired</code> (optionally synchronous)</dt>
   <dd>Fires when a request requires authentication of the user. This event can
   be handled synchronously to provide authentication credentials. Note that
   extensions may provide invalid credentials. Take care not to enter an infinite
   loop by repeatedly providing invalid credentials.</dd>
   <dt><code>onBeforeRedirect</code></dt>
   <dd>Fires when a redirect is about to be executed. A redirection can be
   triggered by an HTTP response code or by an extension. This event is
   informational and handled asynchronously. It does not allow you to modify or
   cancel the request. </dd>
@@ skipping 220 matching lines @@
       }
     }
     return {requestHeaders: details.requestHeaders};
   },
   {urls: ["&lt;all_urls&gt;"]},
   ["blocking", "requestHeaders"]);
 </pre>
 
 <p> For more example code, see the <a href="samples.html#webrequest">web request
 samples</a>.</p>