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

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

 'use strict';
 
 class CompositedAnimationTestCommon {
   constructor(composited) {
     this.composited = composited;
     this.tests = [];
     this.nextInstanceId = 1;
 
     this.createStyles();
     this.createStaticElements();
@@ skipping 121 matching lines @@
     });
   }
 
   reportError(test, message) {
     if (!this.error.textContent)
       this.error.textContent = `${this.composited ? 'Tests:' : 'TestExpectations:'} `;
 
     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();
+      });
     });
   }
 
   registerTestsData(testSuiteData) {
     this.suiteStyle = testSuiteData.style;
     for (var testName in testSuiteData.tests) {
       var testData = testSuiteData.tests[testName];
       this.tests.push({
         name: testName,
         data: testData,
@@ skipping 39 matching lines @@
 }
 
 var getLinearSamples = function(n, start, end) {
   var arr = [];
   var spread = end - start;
   for (var i = 0; i <= n; i++)
     arr.push(i * spread / n + start);
   return arr.map(t => { return {at: t} });
 }