Allow performance tests to test full rendering performance

PerformanceTests/ShadowDOM/ChangingSelect.html

Index: PerformanceTests/ShadowDOM/ChangingSelect.html
diff --git a/PerformanceTests/ShadowDOM/ChangingSelect.html b/PerformanceTests/ShadowDOM/ChangingSelect.html
index a7633ad4024c811e18553fbccb050ed20d16ad7e..19a1ccea281d4533f8175fae3a232ad0f8870f37 100644
--- a/PerformanceTests/ShadowDOM/ChangingSelect.html
+++ b/PerformanceTests/ShadowDOM/ChangingSelect.html
@@ -42,9 +42,7 @@ function run()
     for (var i = 0; i < 100; ++i) {
         for (var j = 0; j < typeNames.length; ++j) {
             contentSelected.select = '.' + typeNames[j];
-
-            selected.offsetLeft;
-            nonSelected.offsetLeft;

[Julien - ping for review, 2014/11/17 19:05:46]

It's bit concerning that if people are not careful, they will just miss out our
function. Unfortunately offsetLeft / offsetTop / clientHeight are read-only
attributes so we can't just modify them on body to alleviate the issue.

+            PerfTestRunner.layoutOrFullyRender();
         }
     }
 }