Reland Extension Installed InfoBubble

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

 <div id="pageData-title" 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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   "default_icon": "icons/foo.png" <em>// optional</em>
 }</pre>
 
 <p><b>Note:</b> Even if you don't include
   any of the values,
   you must include an empty <b>page_action</b> entry
   in your manifest so that the system
   reserves a slot for your page action
   in the address bar.</p>
 
+<p>
+Although the "default_icon" is optional, it's a good idea to provide one.
+When users install your extension, if you have set a default icon, it can
+be previewed to the user so they know what to look for when your page action
+is active for a given page.</p>
+
 <h2 id="ui">Parts of the UI</h2>
 
 <p>
 Page actions have an icon and optional tooltip,
 just like browser actions do.
 In addition, page actions can appear and disappear.
 For information about the common UI parts,
 read about the
 <a href="browserAction.html#ui">browser action UI</a>.
 </p>
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     in the location bar
     are smaller than 19 pixels.
     If the edge pixels are used,
     they are usually only used
     for a faint shadow.
   <li><b>Don't</b> constantly animate your icon.
     That's just annoying.
 </ul>
 
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