Update write checks for external extension file on mac.

chrome/common/extensions/docs/static/external_extensions.html

 <div id="pageData-name" class="pageData">Other Deployment Options</div>
 <div id="pageData-showTOC" class="pageData">true</div>
 
 <p>
 Usually, users install their own extensions.
 But sometimes you might want an extension
 to be installed automatically.
 Here are two typical cases:
 </p>
 
@@ skipping 85 matching lines @@
   If the file doesn't exist, create it.
   The location depends on the operating system.
   <dl>
   <dt> Windows: </dt>
     <dd> <code><em>chrome_root</em>\Application\<em>chrome_version</em>\Extensions\</code>
     <br />
     Example: <code>c:\Users\Me\AppData\Local\Google\Chrome\Application\6.0.422.0\Extensions\</code>
     </dd>
   <dt> Mac OS X: </dt>
     <dd> <code>/Library/Application Support/Google/Chrome/External Extensions/</code> </dd>
-    <p class="note"><b>Note:</b> The external extensions file is read only if every directory in the path is owned by the user <code>root</code>, has the group <code>admin</code>, and is not world readable.  The path must also be free of symbolic links.  These restrictions prevent an unprivileged user from causing extensions to be installed for all users.  See <a href="#troubleshooting">troubleshooting</a> for details.</p>
+    <p class="note"><b>Note:</b> The external extensions file is read only if every directory in the path is owned by the user <code>root</code>, has the group <code>admin</code> or <code>wheel</code>, and is not world readable.  The path must also be free of symbolic links.  These restrictions prevent an unprivileged user from causing extensions to be installed for all users.  See <a href="#troubleshooting">troubleshooting</a> for details.</p>

[Mark Mentovai, 2011/10/17 18:53:57]

As before.

[Sam Kerner (Chrome), 2011/10/17 19:35:16]

Done.

     <p class="note">
     <b>Note:</b> The above path was added in Chrome 16.  Prior versions used a different path, which is now deprecated:
     <code>/Applications/Google Chrome.app/Contents/Extensions/</code> .</p>
   <dt> Linux: </dt>
     <dd> <code>/opt/google/chrome/extensions/</code> <br>
     <b>Note:</b> Use <code>chmod</code> if necessary
     to make sure that <code>extensions/external_extensions.json</code>
     is world-readable.
     </dd>
   </dl>
@@ skipping 179 matching lines @@
 
 <br>
 
 <p><b>How do I get off the blacklist?</b> </p>
 <p>If the user uninstalls your extension, you should respect that
 decision. However, if you (the developer) accidentally uninstalled
 your extension through the UI,
 you can remove the blacklist tag
 by installing the extension normally
 through the UI, and then uninstalling it. </p>