Bug 1391803 - Use nsStringFwd.h for forward declaring string classes. r=froydnj

We should not be declaring forward declarations for nsString classes directly,
instead we should use nsStringFwd.h. This will make changing the underlying
types easier.

dom/html/nsIHTMLCollection.h

@@ -8,13 +8,13 @@
 #define nsIHTMLCollection_h___
 
 #include "nsIDOMHTMLCollection.h"
+#include "nsStringFwd.h"
 #include "nsTArrayForwardDeclare.h"
 #include "nsWrapperCache.h"
 #include "js/GCAPI.h"
 #include "js/TypeDecls.h"
 
 class nsINode;
-class nsString;
 
 namespace mozilla {
 namespace dom {