[animation worklet] Fix incorrect use of testharness in a Layouttest

Currently there is a call to rAF inside step_func_done which means the
test will actually complete before the rAF fires and so the assert_true
will never run.

Bug: 887659
Change-Id: Ic3d017cb106994507cba995ccf41133902c0d101
Reviewed-on: https://chromium-review.googlesource.com/c/1333887
Commit-Queue: Yi Gu <yigu@chromium.org>
Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607597}

third_party/WebKit/LayoutTests/animations/animationworklet/worklet-animation-currentTime.html

@@ -31,15 +31,15 @@ runInAnimationWorklet(
   assert_equals(first_opacity, '1');
   animation.play();
 
-  waitTwoAnimationFrames(t.step_func_done(() => {
+  waitTwoAnimationFrames(() => {
     // waitTwoAnimationFrames guarantees a compositor frame that could update
     // the opacity value in the worklet. Meanwhile, getComputedStyle needs an
     // extra frame to fetch the updated value.
-    window.requestAnimationFrame( _ => {
+    window.requestAnimationFrame(t.step_func_done(() => {
       const second_opacity = getComputedStyle(target).opacity;
       assert_true(second_opacity < first_opacity);
-    });
-  }));
+    }));
+  });
 });
 }, 'Opacity should change as the animation starts.');
 </script>