Bug 976272. Use DOMString for HTMLElement.className, and mark it as [Pure]. r=smaug

content/html/content/src/nsGenericHTMLElement.h

@@ -260,7 +260,7 @@ public:
 #undef ERROR_EVENT
 #undef FORWARDED_EVENT
 #undef EVENT
-  void GetClassName(nsString& aClassName)
+  void GetClassName(mozilla::dom::DOMString& aClassName)
   {
     GetAttr(kNameSpaceID_None, nsGkAtoms::_class, aClassName);
   }

dom/webidl/HTMLElement.webidl

@@ -77,6 +77,7 @@ interface HTMLElement : Element {
 
   // Mozilla specific stuff
   // FIXME Bug 810677 Move className from HTMLElement to Element
+  [Pure]
            attribute DOMString className;
 
            attribute EventHandler oncopy;