Add null check for frame when creating WebCore::Performance.

LayoutTests/fast/dom/Window/property-access-on-cached-properties-after-frame-removed-and-gced.html

Index: LayoutTests/fast/dom/Window/property-access-on-cached-properties-after-frame-removed-and-gced.html
diff --git a/LayoutTests/fast/dom/Window/property-access-on-cached-properties-after-frame-removed.html b/LayoutTests/fast/dom/Window/property-access-on-cached-properties-after-frame-removed-and-gced.html
similarity index 78%
copy from LayoutTests/fast/dom/Window/property-access-on-cached-properties-after-frame-removed.html
copy to LayoutTests/fast/dom/Window/property-access-on-cached-properties-after-frame-removed-and-gced.html
index ca82951736706a16a85f083f2ed2ce97d7d424b5..45490b58dabeb1c25957597e892419db62f74790 100644
--- a/LayoutTests/fast/dom/Window/property-access-on-cached-properties-after-frame-removed.html
+++ b/LayoutTests/fast/dom/Window/property-access-on-cached-properties-after-frame-removed-and-gced.html
@@ -3,6 +3,8 @@
 <script src="../../../resources/js-test.js"></script>
 <script src="resources/window-property-collector.js"></script>
 <script>
+var jsTestIsAsync = true;
+
 var childWindow;
 var propertiesToVerify = [];
 
@@ -21,13 +23,21 @@ function insertExpectedResult(path, expected)
 
 function runTest()
 {
-    description("Tests access of cached DOMWindow properties after the associated frame is no longer in a web page. Test should not crash and properties should be set to sane defaults.");
+    description("Tests access of cached DOMWindow properties after the associated frame is removed from a web page and garbage collected. Test should not crash and properties should be set to sane defaults.");
     var frame = document.getElementById("frame");
     childWindow = frame.contentWindow;
     collectProperties();
     frame.parentNode.removeChild(frame);
+    window.setTimeout(verifyResults, 0);
+}
+
+function verifyResults()
+{
+    if (window.gc)
+        gc();
     for (var i = 0; i < propertiesToVerify.length; ++i)
         shouldBe(propertiesToVerify[i].property, propertiesToVerify[i].expected);
+    finishJSTest();
 }
 </script>
 </head>