Enable error pages for layout tests.

third_party/WebKit/LayoutTests/fast/xmlhttprequest/null-document-xmlhttprequest-open.html

-<p>This test checks for a NULL document crash that can happen when calling
-XMLHttpRequest.open. If the test passes, you'll see a PASS message below.</p>
-<hr>
-<pre id="pre"></pre>
-<iframe name="iframe" style="visibility:hidden" src="does-not-exist.bogus"></iframe> <!-- forces asynchronous load -->
-<script>
-<!--
-function log(s)
-{
-    document.getElementById("pre").appendChild(document.createTextNode(s));
-}
-
-function test()
-{
-    var request = new XMLHttpRequest();
-    request.onreadystatechange = function(event) { if (event.target.readyState == 1) parent.pass(); };
-    request.open("GET", window.location);
-}
-
-function pass()
-{
-    log("PASS: You didn't crash.");
-
-    if (window.testRunner)
-        testRunner.notifyDone();
-}
-
-function main()
-{
-    if (window.testRunner) {
-        testRunner.dumpAsText();
-        testRunner.waitUntilDone();
-    }    
-    
-    // setTimeout forces execution in the context of the frame
-    frames[0].setTimeout(test, 0);
-}
-
-main();
--->
-</script>