Paint some content for animated elements in composited animation tests.

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 16 matching lines @@
       background: orange;
       margin: 15px;
     }`;
 
     document.head.appendChild(styleSheet);
   }
 
   createStaticElements() {
     this.error = document.createElement('span');
     this.error.style.color = 'red';
+    // The element must have some painted content in order to be composited.
+    this.error.textContent = 'x';
     document.body.appendChild(this.error);
 
     this.wrapper = document.createElement('div');
     document.body.appendChild(this.wrapper);
   }
 
   createTestElements() {
     this.tests.forEach(test => {
       test.testWrapper = document.createElement('div');
       this.wrapper.appendChild(test.testWrapper);
@@ skipping 159 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} });
 }