DOMException toString is not correct

LayoutTests/fast/dom/HTMLDialogElement/dialog-show-modal.html

Index: LayoutTests/fast/dom/HTMLDialogElement/dialog-show-modal.html
diff --git a/LayoutTests/fast/dom/HTMLDialogElement/dialog-show-modal.html b/LayoutTests/fast/dom/HTMLDialogElement/dialog-show-modal.html
index bc99321ace9783e7141d14bdfbe52301e70a018d..0e7c8712930b3e28cd7016e6cd8cc1bfdc605669 100644
--- a/LayoutTests/fast/dom/HTMLDialogElement/dialog-show-modal.html
+++ b/LayoutTests/fast/dom/HTMLDialogElement/dialog-show-modal.html
@@ -21,14 +21,14 @@ shouldBeEqualToString("computedStyle.getPropertyValue('display')", "block");
 
 // The quoted texts output below are from <http://www.whatwg.org/specs/web-apps/current-work/multipage/commands.html#dom-dialog-showmodal>.
 debug('"If dialog already has an open attribute, then throw an InvalidStateError exception."');
-shouldThrow('dialog.showModal();', "'Error: InvalidStateError: DOM Exception 11'");
+shouldThrow('dialog.showModal();', "'InvalidStateError: An attempt was made to use an object that is not, or is no longer, usable.'");
 
 dialog.close();
 shouldBeEqualToString("computedStyle.getPropertyValue('display')", "none");
 
 dialog.parentNode.removeChild(dialog);
 debug('"If dialog is not in a Document, then throw an InvalidStateError exception."');
-shouldThrow('dialog.showModal();', "'Error: InvalidStateError: DOM Exception 11'");
+shouldThrow('dialog.showModal();', "'InvalidStateError: An attempt was made to use an object that is not, or is no longer, usable.'");
 
 doc = document.implementation.createHTMLDocument();
 doc.body.appendChild(dialog);