Bug 1603933 - Use Date.now() instead of DocumentTimeline.currentTime. r=tnikkel

Differential Revision: https://phabricator.services.mozilla.com/D57213

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

image/test/mochitest/test_animated_gif.html

@@ -35,11 +35,11 @@
       iframe.src = "http://example.org/tests/image/test/mochitest/child.html";
     });
 
-    const start = document.timeline.currentTime;
+    const start = Date.now();
 
     // Waits a second;
     await SimpleTest.promiseWaitForCondition(() => {
-      return 1000 < (document.timeline.currentTime - start);
+      return 1000 < (Date.now() - start);
     });
 
     ok(true, "decodeComplete didn't receive within a second");