Run headless chromium with --hide-scrollbars

This patch runs chromium in headless mode with the --hide-scrollbars
flag so that it doesn't spoil screenshots.

Fixes #68.

test/assets/grid.html

@@ -28,11 +28,6 @@ document.addEventListener('DOMContentLoaded', function() {
 
 <style>
 
-/* Hide scrollbar so that it is not captured on screenshots */
-::-webkit-scrollbar {
-    display: none;
-}
-
 body {
     margin: 0;
     padding: 0;