Add new user script injection point: document_idle.

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

 <div id="pageData-title" class="pageData">Content Scripts</div>
 <div id="pageData-showTOC" class="pageData">true</div>
 
 <p>
 Content scripts are JavaScript files that run in the context of web pages.
 By using the standard
 <a href="http://www.w3.org/TR/DOM-Level-2-HTML/">Document
 Object Model</a> (DOM),
 they can read details of the web pages the browser visits,
 or make changes to them.
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     <td>Optional. The list of JavaScript files to be injected into matching pages. These are injected in the order they appear in this array.</td>
   </tr>
   <tr>
     <td>css</td>
     <td>array of strings</td>
     <td>Optional. The list of CSS files to be injected into matching pages. These are injected in the order they appear in this array, before any DOM is constructed or displayed for the page.</td>
   </tr>
   <tr>
     <td>run_at</td>
     <td>string</td>
-    <td>Optional. Controls when the files in <code>js</code> are injected. Can be <code>"document_start"</code> or <code>"document_end"</code>. Defaults to <code>"document_end"</code>. In the case of <code>"document_start"</code>, the files are injected after any files from <code>"css"</code>, but before any other DOM is constructed or any other script is run. In the case of <code>"document_end"</code>, the files are injected after the DOM is complete, but before subresources like images and frames have necessarily loaded.</td>
+    <td>Optional. Controls when the files in <code>js</code> are injected. Can be <code>"document_start"</code>, <code>"document_end"</code>, or <code>"document_idle"</code>. Defaults to <code>"document_idle"</code>.
+    
+    <br><br>
+    
+    In the case of <code>"document_start"</code>, the files are injected after any files from <code>"css"</code>, but before any other DOM is constructed or any other script is run.
+    
+    <br><br>
+    
+    In the case of <code>"document_end"</code>, the files are injected immediately after the DOM is complete, but before subresources like images and frames have loaded.
+    
+    <br><br>
+    
+    In the case of <code>"document_idle"</code>, the browser chooses a time to inject scripts between <code>"document_end"</code> and immediately after the <code><a href="http://www.whatwg.org/specs/web-apps/current-work/#handler-onload">window.onload</a></code> event fires. The exact moment of injection depends on how complex the document is and how long it is taking to load, and is optimized for page load speed.
+    
+    <br><br>
+    
+    <b>NOTE:</b> In <code>document_idle</code>, content scripts may not necessarily receive the window.onload event, because they may run after it has
+    already fired. In most cases, listening for the onload event is unnecessary for content scripts running at <code>document_idle</code> because they are guaranteed to run after the DOM is complete. If your script definitely needs to run after <code>window.onload</code> you can check if it has already fired by using the <code><a href="http://www.whatwg.org/specs/web-apps/current-work/#dom-document-readystate">document.readyState</a></code> property.</td>
   </tr>
 </table>
 
 
 <h2 id="execution-environment">Execution environment</h2>
 
 <p>Content scripts execute in a special environment called an <em>isolated world</em>. They have access to the DOM of the page they are injected into, but not to any JavaScript variables or functions created by the page. It looks to each content script as if there is no other JavaScript executing on the page it is running on. The same is true in reverse: JavaScript running on the page cannot call any functions or access any variables defined by content scripts.
 
 <p>For example, consider this simple page:
 
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 just like you would any other URL,
 as the following code shows.
 </p>
 
 
 <pre>
 <em>//Code for displaying &lt;extensionDir>/images/myimage.png:</em>
 var imgURL = <b>chrome.extension.getURL("images/myimage.png")</b>;
 document.getElementById("someImage").src = imgURL;
 </pre>