Add safePrintableInset parameter for printing.

This is for testing the `page-margin-safety` descriptor in `@page` and
page margin box contexts.

Spec discussion: w3c/csswg-drafts#11395
Spec change: w3c/csswg-drafts#13190
RFC: web-platform-tests/rfcs#233

wptrunner and webdriver code changes:
web-platform-tests/wpt#58030

Author: mstensho

index.html

@@ -11313,6 +11313,14 @@ <h3><dfn>Print Page</dfn></h3>
  <li><p>If <var>shrinkToFit</var> is not a <a>Boolean</a>
  return <a>error</a> with <a>error code</a> <a>invalid argument</a>.
 
+ <li><p>Let <var>safePrintableInset</var> be the result of <a>getting a
+  property with default</a> named "<code>safePrintableInset</code>" and with
+  default <code>0</code> from <var>parameters</var>.
+
+ <li><p>If <var>safePrintableInset</var> is not a <a>Number</a>, or is a
+  negative number, return <a>error</a> with <a>error code</a> <a>invalid
+  argument</a>.
+
  <li><p>Let <var>pageRanges</var> be the result of <a>getting a
   property with default</a> named "<code>pageRanges</code>" from
   <var>parameters</var> with default of an