Bug 1402450 - Ensure the test properly delays until the content process receives the touch events. r=botond

It appears that in this case just waiting for the APZ flush to complete was
insufficient, probably because the APZ flush and touch inputs go via different
IPC channels and therefore do not provide ordering guarantees. So instead of
waiting for the APZ flush, let's just wait until the touchstart is received
in the content process and use that to resume the test.

MozReview-Commit-ID: AcPRhox1Xkg

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

@@ -13,8 +13,11 @@ function failure(e) {
   ok(false, "This event listener should not have triggered: " + e.type);
 }
 
-function success(e) {
-  success.triggered = true;
+function listener(callback) {
+  return function(e) {
+    ok(e.type == 'touchstart', "The touchstart event handler was triggered after snapshotting completed");
+    setTimeout(callback, 0);
+  };
 }
 
 // This helper function provides a way for the child process to synchronously
@@ -220,7 +223,7 @@ function* test(testDriver) {
   ok(numDifferentSnapshotPairs > 0,
      "The number of different snapshot pairs was " + numDifferentSnapshotPairs);
 
-  // Wait for the touchend as well, just for good form
+  // Wait for the touchend as well, to clear all pending testDriver resumes
   yield;
   ok(waitFor('touchend', 1), "Touchend processed in chrome process");
 
@@ -231,10 +234,11 @@ function* test(testDriver) {
   // so that all the DOM events that were queued up can be processed. We
   // register a touchstart listener to make sure this happens.
   document.body.removeEventListener('touchstart', failure);
-  document.body.addEventListener('touchstart', success, { passive: true });
-  yield flushApzRepaints(testDriver);
-  ok(success.triggered, "The touchstart event handler was triggered after snapshotting completed");
-  document.body.removeEventListener('touchstart', success);
+  var listenerFunc = listener(testDriver);
+  document.body.addEventListener('touchstart', listenerFunc, { passive: true });
+  dump('done registering listener, going to yield\n');
+  yield;
+  document.body.removeEventListener('touchstart', listenerFunc);
 }
 
 if (SpecialPowers.isMainProcess()) {