Fix flaky compositing/reflections/nested-reflection-transition.html

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

 /* This is the helper script for animation tests:
 
 Test page requirements:
 - The body must contain an empty div with id "result"
 - Call this function directly from the <script> inside the test page
 
 runAnimationTest and runTransitionTest parameters:
     expected [required]: an array of arrays defining a set of CSS properties that must have given values at specific times (see below)
     callbacks [optional]: a function to be executed immediately after animation starts;
                           or, an object in the form {time: function} containing functions to be
@@ skipping 149 matching lines @@
 
     if (computedNumbers.length !== expectedNumbers.length)
         return false;
     for (var i = 0; i < expectedNumbers.length; i++)
         if (!isCloseEnough(computedNumbers[i], expectedNumbers[i], tolerance))
             return false;
 
     return true;
 }
 
+function waitForCompositor() {
+    return document.body.animate({opacity: [1, 1]}, 1).finished;
+}
+
 function endTest()
 {
     log('Ending test');
     var resultElement = useResultElement ? document.getElementById('result') : document.documentElement;
     if (ENABLE_ERROR_LOGGING && result.indexOf('FAIL') >= 0)
         result += '<br>Log:<br>' + logMessages.join('<br>');
     resultElement.innerHTML = result;
 
-    if (window.testRunner)
-        testRunner.notifyDone();
+    if (window.testRunner) {
+        waitForCompositor().then(() => {
+            testRunner.notifyDone();
+        });
+    }
 }
 
 function runChecksWithRAF(checks)
 {
     var finished = true;
     var time = performance.now() - animStartTime;
 
     log('RAF callback, animation time: ' + time);
     for (var k in checks) {
         var checkTime = Number(k);
@@ skipping 118 matching lines @@
         var target = isTransitionsTest ? window : document;
         var event = isTransitionsTest ? 'load' : 'webkitAnimationStart';
         target.addEventListener(event, begin, false);
     }
 }
 
 function runTransitionTest(expected, trigger, callbacks, doPixelTest, disablePauseAnimationAPI) {
     isTransitionsTest = true;
     runAnimationTest(expected, callbacks, trigger, disablePauseAnimationAPI, doPixelTest);
 }