Clean up testing.internals argument type checking.

LayoutTests/fast/dom/shadow/layout-tests-can-access-shadow.html

Index: LayoutTests/fast/dom/shadow/layout-tests-can-access-shadow.html
diff --git a/LayoutTests/fast/dom/shadow/layout-tests-can-access-shadow.html b/LayoutTests/fast/dom/shadow/layout-tests-can-access-shadow.html
index 3cb562baa01beb9f41e623d8eb2b86c54b099cc3..aed236bdd1fdc688e554abd053322597fe4f0c24 100644
--- a/LayoutTests/fast/dom/shadow/layout-tests-can-access-shadow.html
+++ b/LayoutTests/fast/dom/shadow/layout-tests-can-access-shadow.html
@@ -17,15 +17,8 @@ if (window.internals) {
     var shadow = internals.shadowRoot(keygen);
     shouldBe('shadow.nodeName', '"#document-fragment"');
 
-    // Shadow roots aren't elements, so accessing their shadow should
-    // raise
-    var exceptionCode;
-    try {
-        internals.shadowRoot(shadow);
-    } catch (e) {
-        exceptionCode = e.code;
-    }
-    shouldBe('exceptionCode', 'DOMException.INVALID_ACCESS_ERR');
+    // Shadow roots aren't elements, so accessing their shadow should throw.
+    shouldThrow("internals.shadowRoot(shadow)");
 
     // Ordinary element should not have shadow
     var p = document.getElementById('peas');