Bug 1599619: Wait for a paint before making dynamic change in SMIL reftest "anim-css-font-1.svg". r=boris

This small change seems to help us come out on the good side of a race
condition that this test is failing on our Ubuntu 18.04 test runner platform.

Differential Revision: https://phabricator.services.mozilla.com/D54974

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

layout/reftests/svg/smil/style/anim-css-font-1.svg

@@ -2,8 +2,13 @@
      xmlns:xlink="http://www.w3.org/1999/xlink"
      width="300px" height="400px"
      class="reftest-wait"
-     onload="setTimeAndSnapshot(3.0, true)">
+     onload="go()">
   <script xlink:href="../smil-util.js" type="text/javascript"/>
+  <script>
+    function go() {
+      requestAnimationFrame(() => { setTimeAndSnapshot(3.0, true) });
+    }
+  </script>
 
   <!-- 'inherit' to 'caption' -->
   <text x="0px"   y="50px">abc