Allow performance tests to test full rendering performance

PerformanceTests/Layout/resources/character_fallback_test.js

 // Perf Tests run a maximum of 20 times,
 // make sure we have an equal amount of characters
 // for each run.
 var selectionSize = fallbackChars.length / 21;
 var target;
 
 function test() {
     var charSelection = "";
     for(var i=0; i < selectionSize; i++) {
         var selectedCharIndex = Math.floor(Math.random() * fallbackChars.length);
         if(!fallbackChars[selectedCharIndex])
             continue;
         charSelection += fallbackChars[selectedCharIndex];
         fallbackChars.splice(selectedCharIndex, 1);
     }
     if (charSelection.length)
         replaceTextAndWaitForLayout(charSelection);
 }
 
 function replaceTextAndWaitForLayout(charSelection) {
     while (target.firstChild)
         target.removeChild(target.firstChild);
     target.appendChild(document.createTextNode(charSelection));
-    target.offsetHeight;
+    PerfTestRunner.forceLayoutOrFullFrame();
 }
 
 function cleanup() {
     replaceTextAndWaitForLayout("");
 }
 
 function startTest() {
     target = document.querySelector("#target");
     PerfTestRunner.measureTime({ run: test, done: cleanup, description: "Per-character font fallback" });
 }