CSS Animations: Fix flaky composited-animations-simple.html

third_party/WebKit/LayoutTests/animations/resources/composited-animation-test.js

Index: third_party/WebKit/LayoutTests/animations/resources/composited-animation-test.js
diff --git a/third_party/WebKit/LayoutTests/animations/resources/composited-animation-test.js b/third_party/WebKit/LayoutTests/animations/resources/composited-animation-test.js
index dfdbd3a1294bf16649d065f16739f49c88fc02d3..3b5fc0020a4a13fc181186abe5d9fed89c3ed012 100644
--- a/third_party/WebKit/LayoutTests/animations/resources/composited-animation-test.js
+++ b/third_party/WebKit/LayoutTests/animations/resources/composited-animation-test.js
@@ -139,15 +139,21 @@ class CompositedAnimationTestCommon {
     this.error.textContent += `${test.name}: ${message} `;
   }
 
+  waitForCompositor() {
+    return this.error.animate({opacity: ['1', '1']}, 1).ready;
+  }
+
   layoutAndPaint() {
     if (window.testRunner)
       testRunner.waitUntilDone();
 
-    requestAnimationFrame(() => {
-      if (window.internals)
-        this.assertAnimationCompositedState();
-      if (window.testRunner)
-        testRunner.notifyDone();
+    this.waitForCompositor().then(() => {
+      requestAnimationFrame(() => {
+        if (window.internals)
+          this.assertAnimationCompositedState();
+        if (window.testRunner)
+          testRunner.notifyDone();
+      });
     });
   }