Bug 1209942 - Flush APZ repaints before waiting for paints. r=dvander

--HG--
extra : commitid : ANYadL4Yb6u

dom/events/test/test_bug574663.html

@@ -50,7 +50,15 @@ function forceScrollAndWait(scrollbox, callback) {
   let utils = SpecialPowers.getDOMWindowUtils(win);
 
   utils.advanceTimeAndRefresh(1000);
-  waitForPaint(win, utils, callback);
+
+  let postApzFlush = function() {
+    SpecialPowers.Services.obs.removeObserver(postApzFlush, "apz-repaints-flushed", false);
+    waitForPaint(win, utils, callback);
+  }
+  SpecialPowers.Services.obs.addObserver(postApzFlush, "apz-repaints-flushed", false);
+  if (!utils.flushApzRepaints()) {
+    postApzFlush();
+  }
 }
 
 function sendTouchpadScrollMotion(scrollbox, direction, ctrl, momentum, callback) {