Allow performance tests to test full rendering performance

PerformanceTests/Layout/resources/large-table-with-collapsed-borders.js

 (function() {
     function createElement(tag, parent, className, id) {
         var el = document.createElement(tag);
         el.className = className;
         if (id)
             el.id = id;
         parent.appendChild(el);
         return el;
     }
 
     function createTable(width, height, colspan) {
         var table = createElement("table", document.body, "table");
         for (var y = 0; y < height; ++y) {
             var tr = createElement("tr", table, "tr");
             for (var x = 0; x < width; ++x) {
                 var td = createElement("td", tr, "td");
                 if (colspan > 0 && x==10 && y==0)
                     table.rows[y].cells[x].colSpan = colspan;
             }
         }
         return table;
     }
 
     function createTestFunction(width, height, colspan) {
         return function() {
             var table = createTable(width, height, colspan);
-            table.clientHeight;
+            PerfTestRunner.forceLayoutOrFullFrame();
         }
     }
 
     window.createTableTestFunction = createTestFunction;
 })();