Add SelectionMode enum for setRangeText() in HTMLInputElement and HTMLTextAreaElement

Source/core/html/HTMLTextAreaElement.idl

Index: Source/core/html/HTMLTextAreaElement.idl
diff --git a/Source/core/html/HTMLTextAreaElement.idl b/Source/core/html/HTMLTextAreaElement.idl
index a937807268b0636357fda157ef961f83db658016..ea7ca760f41738b09f32065c1b6f22482c9260d9 100644
--- a/Source/core/html/HTMLTextAreaElement.idl
+++ b/Source/core/html/HTMLTextAreaElement.idl
@@ -58,11 +58,10 @@ interface HTMLTextAreaElement : HTMLElement {
     attribute long selectionEnd;
     attribute DOMString selectionDirection;
     [RaisesException] void setRangeText(DOMString replacement);
-    // TODO(philipj): The selectionMode argument should be a SelectionMode enum.
     [RaisesException] void setRangeText(DOMString replacement,
                                         unsigned long start,
                                         unsigned long end,
-                                        optional DOMString selectionMode = null);
+                                        optional SelectionMode selectionMode = "preserve");
     // TODO(philipj): The start and end arguments should be unsigned long and
     // should not be optional.
     void setSelectionRange([Default=Undefined] optional long start,