Add Web Animations perf tests to tough_animation_cases page set

tools/perf/page_sets/tough_animation_cases/resources/perftesthelper.js

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 (function(){
 'use strict';
 
 if (window.PerfTestHelper) {
   return;
 }
 window.PerfTestHelper = {};
 
 var randomSeed = 3384413;
 window.PerfTestHelper.random = function() {
     var temp = randomSeed;
     // Robert Jenkins' 32 bit integer hash function.
     temp = ((temp + 0x7ed55d16) + (temp << 12))  & 0xffffffff;
     temp = ((temp ^ 0xc761c23c) ^ (temp >>> 19)) & 0xffffffff;
     temp = ((temp + 0x165667b1) + (temp << 5))   & 0xffffffff;
     temp = ((temp + 0xd3a2646c) ^ (temp << 9))   & 0xffffffff;
     temp = ((temp + 0xfd7046c5) + (temp << 3))   & 0xffffffff;
     temp = ((temp ^ 0xb55a4f09) ^ (temp >>> 16)) & 0xffffffff;
     randomSeed = temp;
     return (randomSeed & 0xfffffff) / 0x10000000;
 };
 
+window.PerfTestHelper.getN = function(defaultN) {
+    var match = /N=(\d+)/.exec(window.location.search);
+    if (match) {
+        return Number(match[1]);
+    }
+    if (typeof defaultN === 'undefined') {
+        throw 'Default N value required';
+    }
+    return defaultN;
+};
+
+window.PerfTestHelper.signalReady = function() {
+    console.time('measurementReady');
+    requestAnimationFrame(function() {
+        // FIXME: We must wait at least two frames before
+        // measuring to allow the GC to clean up the
+        // previous test.
+        requestAnimationFrame(function() {
+            window.measurementReady = true;
+            console.timeStop('measurementReady');
+        });
+    });
+};
+
 })();