Allow content script insertion on about:-URLs.

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

 <h1>Match Patterns</h1>
 
 <p>
 <a href="declare_permissions.html#host-permissions">Host
 permissions</a> and <a href="content_scripts.html">content
 script</a> matching are based on a set of URLs defined by
 <dfn>match patterns</dfn>.  A match pattern is essentially a
 URL that begins with a permitted scheme (<code>http</code>,
 <code>https</code>, <code>file</code>, <code>ftp</code>, or
 <code>chrome-extension</code>),
@@ skipping 27 matching lines @@
     or <code>/foo/bar</code>. The path must be present in a
     host permission, but is always treated
     as <code>/*</code>.
   </li>
 </ul>
 
 <p>Here's the basic syntax:</p>
 
 <pre>
 <em>&lt;url-pattern&gt;</em> := <em>&lt;scheme&gt;</em>://<em>&lt;host&gt;</em><em>&lt;path&gt;</em>
-<em>&lt;scheme&gt;</em> := '*' | 'http' | 'https' | 'file' | 'ftp' | 'chrome-extension'
+<em>&lt;scheme&gt;</em> := '*' | 'http' | 'https' | 'file' | 'ftp' | 'chrome-extension' | 'about'
 <em>&lt;host&gt;</em> := '*' | '*.' <em>&lt;any char except '/' and '*'&gt;</em>+
 <em>&lt;path&gt;</em> := '/' <em>&lt;any chars&gt;</em>
 </pre>
 
 <p>
 The meaning of '<code>*</code>' depends on whether
 it's in the <em>scheme</em>, <em>host</em>, or <em>path</em> part.
 If the <em>scheme</em> is <code>*</code>,
 then it matches either <code>http</code> or <code>https</code>,
 and <strong>not</strong> <code>file</code>, <code>ftp</code>,
@@ skipping 116 matching lines @@
   </td>
 
   <td>
     http://mail.google.com/foo/baz/bar<br>
     https://mail.google.com/foobar
   </td>
 </tr>
 
 <tr>
   <td>
+    <code>about:* </code>
+  </td>
+
+  <td>
+    Matches any URL that uses the <code>about</code> scheme.
+    Content scripts matching this pattern are only inserted if the page was
+    loaded by a page whose URL matches at least one of the other declared
+    patterns.

[not at google - send to devlin, 2014/04/18 16:04:55]

what about top-level about: frames?

+  </td>
+
+  <td>
+    about:blank<br>
+    about:srcdoc
+  </td>
+</tr>
+
+<tr>
+  <td>
     <code>chrome-extension://*/* </code>
   </td>
 
   <td>
     Matches any URL pointing to an extension (the first <code>*</code>
     represents a filter for extension IDs, the second for paths).
   </td>
 
   <td>
     chrome-extension://askla...asdf/options.html
@@ skipping 52 matching lines @@
 <tr>
   <td><code>foo://*</code></td>
   <td>Invalid <em>scheme</em></td>
 </tr>
 </tbody>
 </table>
 
 <p>
 Some schemes are not supported in all contexts.
 </p>