Bug 1365761 - Modify the mochitest for bug 1328065 to avoid relying on receiving a mouse event while the mouse is over the scrollbar. r=kats

MozReview-Commit-ID: 5jGiY0n9Hdc

--HG--
extra : rebase_source : fedaf1464966691c117ee76c5a2d1c4a60742c41

gfx/layers/apz/test/mochitest/helper_scroll_over_scrollbar.html

@@ -12,7 +12,8 @@ function* test(testDriver) {
   // scroll over the scrollbar, and make sure the subframe scrolls
   var scrollPos = subframe.scrollTop;
   var scrollbarX = (200 + subframe.clientWidth) / 2;
-  yield moveMouseAndScrollWheelOver(subframe, scrollbarX, 100, testDriver);
+  yield synthesizeNativeWheelAndWaitForScrollEvent(subframe, scrollbarX, 100,
+      0, -10, testDriver);
   ok(subframe.scrollTop > scrollPos, "subframe scrolled after wheeling over scrollbar");
 }