set window rect: allow x/y fields to be signed integers

It should be allowed to set a window's position to negative x/y
coordinates.

Fixes: #804

Author: andreastt

webdriver-spec.html

@@ -3799,17 +3799,20 @@ <h3>Set Window Rect</h3>
   from the <var>parameters</var> argument, else let it be <a><code>null</code></a>.
 
  <li><p>Let <var>x</var> be the result of <a>getting a property</a>
-  named <code>y</code> from the <var>parameters</var> argument,
+  named <code>x</code> from the <var>parameters</var> argument,
   else let it be <a><code>null</code></a>.
 
  <li><p>Let <var>y</var> be the result of <a>getting a property</a>
-  named <code>x</code> from the <var>parameters</var> argument,
+  named <code>y</code> from the <var>parameters</var> argument,
   else let it be <a><code>null</code></a>.
 
- <li><p>If any of <code>width</code> or <code>height</code> or <code>x</code>
-   or <code>y</code> is neither <code>null</code> nor an integer between 0 and
-   2<sup>64</sup> – 1 (inclusive), return <a>error</a> with <a>error code</a>
-   <a>invalid argument</a>.
+ <li><p>If <var>width</var> or <var>height</var> is neither <a><code>null</code></a>
+  nor an integer from 0 to 2<sup>64</sup> − 1,
+  return <a>error</a> with <a>error code</a> <a>invalid argument</a>.
+
+ <li><p>If <var>x</var> or <var>y</var> is neither <a><code>null</code></a>
+  nor an integer from −(2<sup>63</sup>) to 2<sup>63</sup> − 1,
+  return <a>error</a> with <a>error code</a> <a>invalid argument</a>.
 
  <li><p><a>Fully exit fullscreen</a>.