[css-fonts] Add font-size: xxx-large

Replaces font-size prefix keywords -webkit-xxx-large and -konq-xxx-large.
Similar to <font size="7">

resolves w3c#3907

WPTs to be added by implementations, e.g.
https://crbug.com/966249

Author: ericwilligers

css-fonts-4/Overview.bs

@@ -723,7 +723,7 @@ Font size: the 'font-size' property</h3>
 
             Possible values are:
 
-            <pre class=prod>[ xx-small | x-small | small | medium | large | x-large | xx-large ]</pre>
+            <pre class=prod>[ xx-small | x-small | small | medium | large | x-large | xx-large | xxx-large ]</pre>
 
         <dt><dfn><<relative-size>></dfn>
         <dd>
@@ -809,7 +809,7 @@ Font size: the 'font-size' property</h3>
             <th>large
             <th>x-large
             <th>xx-large
-            <th>&nbsp;
+            <th>xxx-large
         <tbody>
             <tr>
                 <th>scaling factor