Add DateTime string attribute to Time

Also handle datetime content attribute when attribute is absent.

Rephrase terms used in the test: dateTime IDL property, datetime
attribute and datetime content attribute.

Remove failure expectations for time element.

components/script/dom/webidls/HTMLTimeElement.webidl

@@ -4,5 +4,5 @@
 
 // https://html.spec.whatwg.org/multipage/#htmltimeelement
 interface HTMLTimeElement : HTMLElement {
-  //         attribute DOMString dateTime;
+  attribute DOMString dateTime;
 };