Allow performance tests to test full rendering performance

PerformanceTests/Interactive/resources/window-resize.js

 if (window.testRunner)
     testRunner.dumpAsText();
 
 (function() {
     var portrait = { width: window.innerWidth, height: window.innerHeight };
     var landscape = { width: window.innerHeight, height: window.innerWidth };
 
     var run = function() {
         window.resizeTo(portrait.width, portrait.height);
-        document.body.offsetTop;
+        PerfTestRunner.forceLayoutOrFullFrame();
         window.resizeTo(landscape.width, landscape.height);
-        document.body.offsetTop;
+        PerfTestRunner.forceLayoutOrFullFrame();
     };
 
     function onTestDone() {
         var logNode = document.getElementById("log");
         logNode.parentNode.removeChild(logNode);
         document.body.innerHTML = "";
         document.body.appendChild(logNode);
     };
 
     function startTest() {
         PerfTestRunner.measureRunsPerSecond({
             description: "Exercising window resize and following relayout",
             run: run,
             done: onTestDone
         });
     }
 
     document.addEventListener("DOMContentLoaded", startTest);
 })();