Update old syntax for action default_icon to populate 19 and 38px icons

chrome/common/extensions/docs/templates/intros/browserAction.html

 <p>
 In the following figure,
 the multicolored square
 to the right of the address bar
 is the icon for a browser action.
 A popup is below the icon.
 </p>
 
 <img src="{{static}}/images/browser-action.png"
   width="363" height="226" />
@@ skipping 34 matching lines @@
 can lose detail or make it look fuzzy. The old syntax for registering the
 default icon is still supported:
 </p>
 
 <pre data-filename="manifest.json">
 {
   "name": "My extension",
   ...
   <b>"browser_action": {
     ...
-    "default_icon": "images/icon19.png"  <em>// optional</em>
-    <em>// equivalent to "default_icon": { "19": "images/icon19.png" }</em>
+    "default_icon": "images/icon38.png"  <em>// optional</em>
+    <em>// equivalent to "default_icon": { "38": "images/icon38.png" }</em>
   }</b>,
   ...
 }
 </pre>
 
 <h2 id="ui">Parts of the UI</h2>
 
 <p>
 A browser action can have an <a href="#icon">icon</a>,
 a <a href="#tooltip">tooltip</a>,
@@ skipping 88 matching lines @@
 <p>For the best visual impact,
   follow these guidelines:</p>
 
 <ul>
   <li><b>Do</b> use browser actions for features
     that make sense on most pages.
   <li><b>Don't</b> use browser actions for features
     that make sense for only a few pages.
     Use <a href="pageAction">page actions</a> instead.
   <li><b>Do</b> use big, colorful icons that make the most of
-    the 19x19-pixel space.
+    the 38x38-pixel space.
     Browser action icons should seem a little bigger
     and heavier than page action icons.
   <li><b>Don't</b> attempt to mimic
     Google Chrome's monochrome menu icon.
     That doesn't work well with themes, and anyway,
     extensions should stand out a little.
   <li><b>Do</b> use alpha transparency
     to add soft edges to your icon.
     Because many people use themes,
     your icon should look nice
     on a variety of background colors.
   <li><b>Don't</b> constantly animate your icon.
     That's just annoying.
 </ul>
 
 <h2 id="examples"> Examples </h2>
 
 <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">Samples</a>.
 </p>