Suggest browser specific extension in Protocol Extensions. (#326)

This addresses the concern raised in #25418 by suggesting
that browser vendors include a product-specific part in the
extension prefix for protocol extensions.

webdriver-spec.html

@@ -1410,24 +1410,26 @@ <h2>Protocol Extensions</h2>
 
 <aside class=example>
  <p>This might lead to a URL of the form
-  <code>/session/5d376174-36f0-11e5-9b9a-6bdf200a3f7f/<var>moz</var>/<var>context</var></code>,
-  where <var>moz</var> is an <a>extension prefix</a>
-  and the vendor prefix associated with Mozilla,
+  <code>/session/5d376174-36f0-11e5-9b9a-6bdf200a3f7f/<var>ms</var>/<var>edge</var>/<var>context</var></code>,
+  where <var>ms/edge</var> is an <a>extension prefix</a>
+  based on the vendor prefix associated with Microsoft,
   and <var>context</var> the <a>extension command name</a>
-  that, in the context of Firefox, allows a <a>local end</a>
-  to switch between content- and chrome contexts.
-  Requesting this URL will call the <a>extension command</a>’s <a>remote end steps</a>.
+  that, in the context of Edge, allows a <a>local end</a>
+  to switch between browser-specific contexts.
+  Requesting this URL will call the <a>extension command</a>’s 
+  <a>remote end steps</a>.
 </aside>
 
-<p>The <a>remote end</a>’s <dfn>extension prefix</dfn>
- is a lowercased <a>ASCII string</a> that forms a URL path element,
- separating <a>extension commands</a>
- from other <a>commands</a>
- to avoid potential resource conflicts with other implementations.
- It is suggested that vendors use their vendor prefixes
- without additional characters as outlined in [[CSS21]],
- notably in <a href=http://www.w3.org/TR/CSS21/syndata.html#vendor-keywords>section 4.1.2.2 on <i>vendor keywords</i></a>,
- as the name for this path element.
+<p>The <a>remote end</a>’s <dfn>extension prefix</dfn> is a
+ lowercased <a>ASCII string</a> that forms a URL path element,
+ separating <a>extension commands</a> from other <a>commands</a> in
+ order to avoid potential resource conflicts with other
+ implementations.  It is suggested that vendors use their vendor
+ prefixes without additional characters as outlined in [[CSS21]],
+ notably in
+ <a href=http://www.w3.org/TR/CSS21/syndata.html#vendor-keywords>section
+ 4.1.2.2 on <i>vendor keywords</i></a>, as the name for this path
+ element, and include a vendor-chosen browser driver identifier.
 
 </section> <!-- /Protocol Extensions -->
 </section> <!-- /The WebDriver Protocol -->