Allow performance tests to test full rendering performance

PerformanceTests/Layout/fixed-grid-lots-of-data.html

Index: PerformanceTests/Layout/fixed-grid-lots-of-data.html
diff --git a/PerformanceTests/Layout/fixed-grid-lots-of-data.html b/PerformanceTests/Layout/fixed-grid-lots-of-data.html
index 5e0c53192ecc1efde98c33c5891ff5a045eaabe2..4c148b111c5d85a80ce4140c3f0f6edbf2298587 100644
--- a/PerformanceTests/Layout/fixed-grid-lots-of-data.html
+++ b/PerformanceTests/Layout/fixed-grid-lots-of-data.html
@@ -21,14 +21,14 @@ body {
 <script src="../resources/runner.js"></script>
 <script>
 function startTest() {
-    document.body.offsetHeight;
+    PerfTestRunner.layoutOrFullyRender();
 
     var index = 0;
     PerfTestRunner.measureRunsPerSecond({
         description: "Measures performance of layout on a page using CSS grid layout (fixed).",
         run: function() {
             document.body.style.width = ++index % 2 ? "99%" : "98%";
-            document.body.offsetHeight;
+            PerfTestRunner.layoutOrFullyRender();
         }
     });
 }