Drop NotEnumerable for the on* event handler IDL attributes (again)

LayoutTests/fast/dom/script-tests/event-handlers.js

+function getObject(interface) {
+    switch(interface) {
+        case "Element":
+            var e = document.createElementNS("http://example.com/", "example");
+            assert_true(e instanceof Element);
+            assert_false(e instanceof HTMLElement);
+            assert_false(e instanceof SVGElement);
+            return e;
+        case "HTMLElement":
+            var e = document.createElement("html");
+            assert_true(e instanceof HTMLElement);
+            return e;
+        case "HTMLBodyElement":
+            var e = document.createElement("body");
+            assert_true(e instanceof HTMLBodyElement);
+            return e;
+        case "HTMLFormElement":
+            var e = document.createElement("form");
+            assert_true(e instanceof HTMLFormElement);
+            return e;
+        case "HTMLFrameSetElement":
+            var e = document.createElement("frameset");
+            assert_true(e instanceof HTMLFrameSetElement);
+            return e;
+        case "SVGElement":
+            var e = document.createElementNS("http://www.w3.org/2000/svg", "rect");
+            assert_true(e instanceof SVGElement);
+            return e;
+        case "Document":
+            assert_true(document instanceof Document);
+            return document;
+        case "Window":
+            assert_true(window instanceof Window);
+            return window;
+    }
+    assert_unreached();
+}
+function testSet(interface, attribute) {
+    test(function() {
+        var object = getObject(interface);
+        function nop() {}
+        assert_equals(object[attribute], null, "Initially null");
+        object[attribute] = nop;
+        assert_equals(object[attribute], nop, "Return same function");
+        object[attribute] = "";
+        assert_equals(object[attribute], null, "Return null after setting string");
+        object[attribute] = null;
+        assert_equals(object[attribute], null, "Finally null");
+    }, "Set " + interface + "." + attribute);
+}
+function testReflect(interface, attribute) {
+    test(function() {
+        var element = getObject(interface);
+        assert_false(element.hasAttribute(attribute), "Initially missing");
+        element.setAttribute(attribute, "return");
+        assert_equals(element.getAttribute(attribute), "return", "Return same string");
+        assert_equals(typeof element[attribute], "function", "Convert to function");
+        element.removeAttribute(attribute);
+    }, "Reflect " + interface + "." + attribute);
+}
+// Object.propertyIsEnumerable cannot be used because it doesn't
+// work with properties inherited through the prototype chain.
+function getEnumerable(interface) {
+    var enumerable = {};
+    for (var attribute in getObject(interface)) {
+        enumerable[attribute] = true;
+    }
+    return enumerable;
+}
+var enumerableCache = {};
+function testEnumerate(interface, attribute) {
+    if (!(interface in enumerableCache)) {
+        enumerableCache[interface] = getEnumerable(interface);
+    }
+    test(function() {
+        assert_true(enumerableCache[interface][attribute]);
+    }, "Enumerate " + interface + "." + attribute);
+}