Allow extensions to add, remove, or change their page action popup.

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

 <div id="pageData-name" class="pageData">Page Actions</div>
 
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 <p>
 Use page actions to put icons inside the address bar.
 Page actions represent actions
 that can be taken on the current page,
 but that aren't applicable to all pages.
 Some examples:
 </p>
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 <a href="manifest.html">extension manifest</a>
 like this:
 </p>
 
 <pre>{
   "name": "My extension",
   ...
   <b>"page_action": {
     "default_icon": "icons/foo.png", <em>// <b>required</b></em>
     "default_title": "Do action",    <em>// optional; shown in tooltip</em>
-    "popup": "popup.html"            <em>// optional</em>
+    "default_popup": "popup.html"    <em>// optional</em>
   }</b>,
   ...
 }</pre>
 
 <h2 id="ui">Parts of the UI</h2>
 
 <p>
 Like browser actions,
 page actions have an icon and
 can also have a tooltip and popup;
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 <p>
 You can find simple examples of using page actions in the
 <a href="http://src.chromium.org/viewvc/chrome/trunk/src/chrome/common/extensions/docs/examples/api/pageAction/">examples/api/pageAction</a>
 directory.
 For other examples and for help in viewing the source code, see
 <a href="samples.html">Samples</a>.
 </p>
 
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