Bug 1663537. Fix layout/base/tests/test_scroll_event_ordering.html to work with desktop zooming scrollbars. r=kats

sendKey uses our new relative scroll path that does the scroll by the apz. But we don't have any scroll event ordering expectations when apz is doing the scroll. We trade lossing those expectations for smoother apz scrolling.

So change the test to use a method that doesn't use our new relative apz scroll path and goes back to main thread scrolling.

Differential Revision: https://phabricator.services.mozilla.com/D89402

layout/base/tests/test_scroll_event_ordering.html

@@ -44,7 +44,7 @@ function doTest() {
   window.getSelection().collapse(inner.firstChild, 0);
   window.requestAnimationFrame(onFrame);
   d.onscroll = onScroll;
-  sendKey("DOWN");
+  d.scroll(0, 100);
 }
 
 function prepareTest() {