Rewrite the 'window.onerror' tests.

LayoutTests/fast/events/resources/onerror-test.js

+function stripURL(url) {
+    return url ? url.match( /[^\/]+\/?$/ )[0] : url;
+}
+
+function throwException(message) {
+    throw new Error(message ? message : "An exception");
+}
+
+var errorsSeen = 0;
+function dumpOnErrorArgumentValuesAndReturn(returnValue, callback) {
+    window.onerror = function (message, url, line, column) {
+        debug("window.onerror: \"" + message + "\" at " + stripURL(url) + " (Line: " + line + ", Column: " + column + ")");
+
+        if (callback)
+            callback(++errorsSeen);
+        if (returnValue)
+            debug("Returning 'true': the error should not be reported in the console as an unhandled exception.");
+        else
+            debug("Returning 'false': the error should be reported in the console as an unhandled exception.");
+        return returnValue;
+    };
+}
+
+function dumpErrorEventAndAllowDefault(callback) {
+    window.addEventListener('error', function (e) {
+        dumpErrorEvent(e)
+        debug("Not calling e.preventDefault(): the error should be reported in the console as an unhandled exception.");
+        if (callback)
+            callback(++errorsSeen);
+    });
+}
+
+function dumpErrorEventAndPreventDefault(callback) {
+    window.addEventListener('error', function (e) {
+        dumpErrorEvent(e);
+        debug("Calling e.preventDefault(): the error should not be reported in the console as an unhandled exception.");
+        e.preventDefault();
+        if (callback)
+            callback(++errorsSeen);
+    });
+}
+
+var eventPassedToTheErrorListener = null;
+var eventCurrentTarget = null;
+function dumpErrorEvent(e) {
+    debug("Handling '" + e.type + "' event (phase " + e.eventPhase + "): \"" + e.message + "\" at " + stripURL(e.filename) + ":" + e.lineno);
+
+    eventPassedToTheErrorListener = e;
+    eventCurrentTarget = e.currentTarget;
+    shouldBe('eventPassedToTheErrorListener', 'window.event');
+    shouldBe('eventCurrentTarget', 'window');
+    eventPassedToTheErrorListener = null;
+    eventCurrentTarget = null;
+}