[cssom] Use USVString for href attribute of Stylesheet and CSSImportRule

As resolved by CSS WG on 2017-04-19, because they are URLs.

This is consistent with HTML: whatwg/html#1282

cssom/Overview.bs

@@ -809,7 +809,7 @@ The {{StyleSheet}} interface represents an abstract, base style sheet.
 <pre class=idl>
 interface StyleSheet {
   readonly attribute DOMString type;
-  readonly attribute DOMString? href;
+  readonly attribute USVString? href;
   readonly attribute (Element or ProcessingInstruction)? ownerNode;
   readonly attribute StyleSheet? parentStyleSheet;
   readonly attribute DOMString? title;
@@ -1650,7 +1650,7 @@ The <code>CSSImportRule</code> interface represents an <code>@import</code> at-r
 
 <pre class=idl>
 interface CSSImportRule : CSSRule {
-  readonly attribute DOMString href;
+  readonly attribute USVString href;
   [SameObject, PutForwards=mediaText] readonly attribute MediaList media;
   [SameObject] readonly attribute CSSStyleSheet styleSheet;
 };