Document the activeTab extension permission.

chrome/common/extensions/docs/templates/articles/manifest.html

 <h1>Formats: Manifest Files</h1>
 
 
 <p>
 Every extension, installable web app, and theme has a
 <a href="http://www.json.org">JSON</a>-formatted manifest file,
 named <code>manifest.json</code>,
 that provides important information.
 </p>
 
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        Required if the extension wants to interact
        with the code running on pages.
        Many extension capabilities, such as
        <a href="xhr.html">cross-origin XMLHttpRequests</a>,
        <a href="content_scripts.html#pi">programmatically injected
        content scripts</a>, and <a href="cookies.html">the cookies API</a>
        require host permissions. For details on the syntax, see
        <a href="match_patterns.html">Match Patterns</a>.
   </td>
 </tr>
+<tr>
+  <td> "activeTab" </td>
+  <td> Requests that the extension be granted permissions according to the
+       <a href="activeTab.html">activeTab</a> specification.
+  </td>
+</tr>
 <tr id="bg">
   <td> "background" </td>
   <td> <p>
        Makes Chrome start up early and and shut down late,
        so that apps and extensions can have a longer life.
        </p>
 
        <p>
        When any installed hosted app, packaged app, or extension
        has "background" permission, Chrome runs (invisibly)
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 goes into more detail about implementing a sandboxing workflow that enables use
 of libraries that would otherwise have issues executing under extension's
 <a href="../extensions/contentSecurityPolicy.html">default Content Security
 Policy</a>.
 </p>
 
 <p>
 Sandboxed page may only be specified when using
 <a href="#manifest_version"><code>manifest_version</code></a> 2 or above.
 </p>