Add a capability for returning the main PID of the browser

Author: spectranaut

index.html

@@ -1578,6 +1578,13 @@ <h2>Capabilities</h2>
   <td>string
   <td>Identifies the <a>default User-Agent value</a> of the <a>endpoint node</a>.
  </tr>
+
+ <tr>
+  <td>ProcessID
+  <td>"<code>processID</code>"
+  <td>string
+  <td>Identifies the main process ID of the <a>endpoint node</a>.
+ </tr>
 </table>
 
 <section>
@@ -2009,6 +2016,9 @@ <h3>Processing capabilities</h3>
 
    <dt>"<code>userAgent</code>"
    <dd>String containing the <a>default User-Agent value</a>.
+
+   <dt>"<code>processID</code>"
+   <dd>The main PID of the user agent as a <a>number</a>.
   </dl>
 
  <li><p>If <var>flags</var> contains "<code>http</code>", add the