Make print() run synchronously

This was a slip-up in the larger clean-up commit in 61da493, where
"synchronous" was changed to "in parallel". All browsers implement the
synchronous behavior.

Fixes #4686.

Tests: web-platform-tests/wpt#17439

Author: TimothyGu

source

@@ -93914,7 +93914,7 @@ function sendData(data) {
 
   <p>When the <dfn><code data-x="dom-print">print()</code></dfn> method is invoked, if the
   <code>Document</code> is <span>ready for post-load tasks</span>, then the user agent must
-  run the <span>printing steps</span> <span>in parallel</span>. Otherwise, the user agent must only set the
+  run the <span>printing steps</span>. Otherwise, the user agent must only set the
   <dfn>print when loaded</dfn> flag on the <code>Document</code>.</p>