IDL: Treat undefined as missing for optional arguments with defaults

LayoutTests/fast/forms/resources/common-setrangetext.js

 function runTestsShouldPass(tagName, attributes)
 {
     attributes = attributes || {};
     window.element = document.createElement(tagName);
     for (var key in attributes)
         element.setAttribute(key, attributes[key]);
     document.body.appendChild(element);
     debug("<hr>");
     debug("Running tests on " + tagName + " with attributes: " + JSON.stringify(attributes) + "\n");
     debug("setRangeText() with only one parameter.");
@@ skipping 72 matching lines @@
 
     evalAndLog("element.value = '0123456789'");
     evalAndLog("element.setSelectionRange(6, 9)");
     evalAndLog("element.setRangeText('AB', 1, 1, 'invalid')"); // Invalid selectMode values default to preserve.
     shouldBeEqualToString("element.value", "0AB123456789");
     shouldBe("element.selectionStart", "8");
     shouldBe("element.selectionEnd", "11");
 
     evalAndLog("element.value = '0123456789'");
     evalAndLog("element.setSelectionRange(6, 9)");
+    evalAndLog("element.setRangeText('AB', 1, 1, undefined)"); // Undefined selectMode also default to preserve.
+    shouldBeEqualToString("element.value", "0AB123456789");
+    shouldBe("element.selectionStart", "8");
+    shouldBe("element.selectionEnd", "11");
+
+    evalAndLog("element.value = '0123456789'");
+    evalAndLog("element.setSelectionRange(6, 9)");
     evalAndLog("element.setRangeText('A', 1, 3, 'preserve')");
     shouldBeEqualToString("element.value", "0A3456789");
     shouldBe("element.selectionStart", "5");
     shouldBe("element.selectionEnd", "8");
 
     evalAndLog("element.value = '0123456789'");
     evalAndLog("element.setSelectionRange(2, 6)");
     evalAndLog("element.setRangeText('A', 1, 4, 'preserve')");
     shouldBeEqualToString("element.value", "0A456789");
     shouldBe("element.selectionStart", "1");
@@ skipping 46 matching lines @@
         shouldThrow("element.value = '0123456789XYZ'");
     else
         evalAndLog("element.value = '0123456789XYZ'");
     var initialValue = element.value;
     shouldThrow("element.setRangeText('ABC', 0, 0)");
     // setRangeText() shouldn't do anything on non-text form controls.
     shouldBeEqualToString("element.value", initialValue);
 }