[cssom] Serialize >> as >> instead of a space to match WebKit

Author: Simon Pieters

cssom/Overview.bs

@@ -552,9 +552,10 @@ part of the chain of the selector, and finally return
  "<code lt>></code>",
  "<code lt>+</code>",
  "<code lt>~</code>",
+ "<code lt>>></code>",
  "<code lt>||</code>",
  as appropriate, followed by another single SPACE (U+0020) if the combinator was
- not a <a>descendant combinator</a> (whitespace or "<code lt>>></code>"), to <var>s</var>.
+ not whitespace, to <var>s</var>.
 
  <li>If this is the last part of the chain of the selector and there is
  a pseudo-element, append "<code lt>::</code>" followed by the name of the