Bug 1210057. Make test_moz_mouse_pixel_scroll_event.html wait for a paint after adding a apz aware (MozMousePixelScroll) event listener. r=kats

Adding an apz aware event listener will change the event regions on the layer tree, so we want to wait until that has happened before running the test.

dom/events/test/test_moz_mouse_pixel_scroll_event.html

@@ -4,6 +4,7 @@
   <title>Test for MozMousePixelScroll events</title>
   <script type="application/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
   <script type="application/javascript" src="/tests/SimpleTest/EventUtils.js"></script>
+  <script type="application/javascript" src="/tests/SimpleTest/paint_listener.js"></script>
   <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css"/>
   <style>
     .scrollable {
@@ -63,6 +64,8 @@ function* prepareScrollUnits()
   }
   window.addEventListener("MozMousePixelScroll", handler, true);
 
+  yield waitForAllPaints(function () { setTimeout(runTest, 0); });
+
   yield synthesizeWheel(gScrollable128, 10, 10,
                   { deltaMode: WheelEvent.DOM_DELTA_LINE,
                     deltaY: 1.0, lineOrPageDeltaY: 1 });