Bug 1563541 - Force a paint before reading the APZ test data. r=botond

Differential Revision: https://phabricator.services.mozilla.com/D37983

--HG--
extra : moz-landing-system : lando

gfx/layers/apz/test/mochitest/test_bug1253683.html

@@ -49,6 +49,7 @@ if (isApzEnabled()) {
   pushPrefs([["apz.displayport_expiry_ms", 0],
              ["apz.test.logging_enabled", true]])
   .then(waitUntilApzStable)
+  .then(forceLayerTreeToCompositor)
   .then(runContinuation(test))
   .then(SimpleTest.finish);
 }