Implements HTMLInputElement.defaultValue

...and changes SetValue to update the input text instead of the content
attr.

Also includes a comment summarizing everything I currently know with
respect to an input elements checkedness vs its IDL attributes vs its
content attributes.

components/script/dom/htmlinputelement.rs

@@ -223,7 +223,16 @@ impl<'a> HTMLInputElementMethods for JSRef<'a, HTMLInputElement> {
     }
 
     // https://html.spec.whatwg.org/multipage/forms.html#dom-input-value
-    make_setter!(SetValue, "value")
+    fn SetValue(self, value: DOMString) {
+        self.textinput.borrow_mut().set_content(value);
+        self.force_relayout();
+    }
+
+    // https://html.spec.whatwg.org/multipage/forms.html#dom-input-defaultvalue
+    make_getter!(DefaultValue, "value")
+
+    // https://html.spec.whatwg.org/multipage/forms.html#dom-input-defaultvalue
+    make_setter!(SetDefaultValue, "value")
 
     // https://html.spec.whatwg.org/multipage/forms.html#attr-fe-name
     make_getter!(Name)

components/script/dom/webidls/HTMLInputElement.webidl

@@ -38,7 +38,7 @@ interface HTMLInputElement : HTMLElement {
   //         attribute DOMString src;
   //         attribute DOMString step;
            attribute DOMString type;
-  //         attribute DOMString defaultValue;
+           attribute DOMString defaultValue;
   [TreatNullAs=EmptyString] attribute DOMString value;
   //         attribute Date? valueAsDate;
   //         attribute unrestricted double valueAsNumber;