IDL: Treat undefined as missing for optional arguments with defaults

LayoutTests/fast/forms/textarea/textarea-setrangetext-expected.txt

Index: LayoutTests/fast/forms/textarea/textarea-setrangetext-expected.txt
diff --git a/LayoutTests/fast/forms/textarea/textarea-setrangetext-expected.txt b/LayoutTests/fast/forms/textarea/textarea-setrangetext-expected.txt
index b70bb49d12b5e88840f4cf3b579386757e3afb50..9896cce27f059971a013ba89c8a9147c866a62fe 100644
--- a/LayoutTests/fast/forms/textarea/textarea-setrangetext-expected.txt
+++ b/LayoutTests/fast/forms/textarea/textarea-setrangetext-expected.txt
@@ -83,6 +83,12 @@ PASS element.selectionStart is 8
 PASS element.selectionEnd is 11
 element.value = '0123456789'
 element.setSelectionRange(6, 9)
+element.setRangeText('AB', 1, 1, undefined)
+PASS element.value is "0AB123456789"
+PASS element.selectionStart is 8
+PASS element.selectionEnd is 11
+element.value = '0123456789'
+element.setSelectionRange(6, 9)
 element.setRangeText('A', 1, 3, 'preserve')
 PASS element.value is "0A3456789"
 PASS element.selectionStart is 5
@@ -211,6 +217,12 @@ PASS element.selectionStart is 8
 PASS element.selectionEnd is 11
 element.value = '0123456789'
 element.setSelectionRange(6, 9)
+element.setRangeText('AB', 1, 1, undefined)
+PASS element.value is "0AB123456789"
+PASS element.selectionStart is 8
+PASS element.selectionEnd is 11
+element.value = '0123456789'
+element.setSelectionRange(6, 9)
 element.setRangeText('A', 1, 3, 'preserve')
 PASS element.value is "0A3456789"
 PASS element.selectionStart is 5