Bug 1784757 - Wait for one frame before asserting restyling state. r=hiro

Differential Revision: https://phabricator.services.mozilla.com/D194209

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

@@ -92,6 +92,9 @@ async function test() {
   // overflow: hidden style in the out-of-process iframe.
   await setup_in_oopif();
 
+  // Wait for a frame to make sure animations are properly started.
+  await observe_styling_in_oopif(1);
+
   let restyleCount = await observe_styling_in_oopif(5);
   is(restyleCount, 0,
      "Animation in an out-of-process iframe which is initially clipped out " +