Fix flaky animations/cross-fade-background-image.html

This CL is intended to fix a flaky layout test.

I cannot repro the flakiness locally, so this would be a
speculative. We suspect that when the test ends, even
though the text matches exactly, the rendering of the
result is not updated yet so we need another rendering
frame to show the correct result.

Bug: 623434
Change-Id: Ib1a521114d31db9d698929761d9e4cd928798e60
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2644722Reviewed-by: Kevin Ellis <kevers@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#846302}

third_party/blink/web_tests/animations/resources/animation-test-helpers.js

@@ -216,7 +216,11 @@ function runChecksWithPauseAPI(checks) {
         internals.pauseAnimations(timeMs / 1000);
         checks[k].forEach(function(check) { check(); });
     }
-    endTest();
+    requestAnimationFrame(function() {
+      requestAnimationFrame(function() {
+        endTest();
+      });
+    });
 }
 
 function startTest(checks)