[cssom] Minor tweak.

Once you've serialized one shorthand that contains `property`, you can
skip to the declaration loop, as the rest of the iterations of
`shorthand loop` are useless.

Author: emilio

cssom-1/Overview.bs

@@ -2095,8 +2095,8 @@ To <dfn export>serialize a CSS declaration block</dfn> <var>declaration block</v
      <var>current longhands</var> have their <a for="CSS declaration">important flag</a> set.
      <li>Append <var>serialized declaration</var> to <var>list</var>.
      <li>Append the property names of all items of <var>current longhands</var> to <var>already serialized</var>.
+     <li>Continue with the steps labeled <i>declaration loop</i>.
     </ol>
-   <li>If <var>property</var> is in <var>already serialized</var>, continue with the steps labeled <i>declaration loop</i>.
    <li>Let <var>value</var> be the result of invoking <a>serialize a CSS value</a> of <var>declaration</var>.
    <li>Let <var>serialized declaration</var> be the result of invoking <a>serialize a CSS declaration</a> with property name <var>property</var>, value
    <var>value</var>, and the <i>important</i> flag set if <var>declaration</var> has its <a for="CSS declaration">important