Update page_action and browser_action docs to say that default_icon is optional not required.

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

 <div id="pageData-name" class="pageData">Browser Actions</div>
 
 <!-- BEGIN AUTHORED CONTENT -->
 <p>Use browser actions to put icons
   in the main Google Chrome toolbar,
   to the right of the address bar.
   In addition to its <a href="#icon">icon</a>,
   a browser action can also have
   a <a href="#tooltip">tooltip</a>,
   a <a href="#badge">badge</a>,
@@ skipping 29 matching lines @@
 <p>
 Register your browser action in the
 <a href="manifest.html">extension manifest</a>
 like this:
 </p>
 
 <pre>{
   "name": "My extension",
   ...
   <b>"browser_action": {
-    "default_icon": "images/icon19.png", <em>// <b>required</b></em>
+    "default_icon": "images/icon19.png", <em>// optional</em>
     "default_title": "Google Mail",      <em>// optional; shown in tooltip</em>
     "default_popup": "popup.html"        <em>// optional</em>
   }</b>,
   ...
 }</pre>
 
 <h2 id="ui">Parts of the UI</h2>
 
 <p>
-A browser action must have an <a href="#icon">icon</a>.
-It can also have
+A browser action can have an <a href="#icon">icon</a>,
 a <a href="#tooltip">tooltip</a>,
 a <a href="#badge">badge</a>,
 and a <a href="#popups">popup</a>.
 </p>
 
 <h3 id="icon">Icon</h3>
 
 <p>Browser action icons can be up to 19 pixels wide and high.
   Larger icons are resized to fit, but for best results,
   use a 19-pixel square icon.</p>
@@ skipping 97 matching lines @@
 
 <p>
 You can find simple examples of using browser actions in the
 <a href="http://src.chromium.org/viewvc/chrome/trunk/src/chrome/common/extensions/docs/examples/api/browserAction/">examples/api/browserAction</a>
 directory.
 For other examples and for help in viewing the source code, see
 <a href="samples.html">Samples</a>.
 </p>
 
 <!-- END AUTHORED CONTENT -->