Make length property return useful values for DOM bindings functions

LayoutTests/fast/js/function-length-expected.txt

+This tests the length property of functions.
+
+On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
+
+
+PASS window.confirm.length is 0
+PASS window.open.length is 2
+PASS window.showModalDialog.length is 1
+PASS window.setTimeout.length is 1
+PASS window.clearTimeout.length is 0
+PASS window.addEventListener.length is 2
+PASS Array.prototype.concat.length is 1
+PASS Array.prototype.join.length is 1
+PASS Array.prototype.push.length is 1
+PASS Array.prototype.slice.length is 2
+PASS Date.UTC.length is 7
+PASS Date.prototype.setSeconds.length is 2
+PASS Date.prototype.setMinutes.length is 3
+PASS Date.prototype.setHours.length is 4
+PASS Date.prototype.setMonth.length is 2
+PASS Date.prototype.setFullYear.length is 3
+PASS Number.prototype.toExponential.length is 1
+PASS Number.prototype.toFixed.length is 1
+PASS Number.prototype.toPrecision.length is 1
+PASS Math.max.length is 2
+PASS Math.min.length is 2
+PASS Function.prototype.apply.length is 2
+PASS Function.prototype.call.length is 1
+PASS String.prototype.split.length is 2
+PASS String.prototype.substring.length is 2
+PASS successfullyParsed is true
+
+TEST COMPLETE
+