Bug 1453828. Fix IsContainerNode to not claim true for doctype nodes. r=mccr8

MozReview-Commit-ID: BC1iBI38wfJ

dom/base/nsINode.h

@@ -434,7 +434,7 @@ public:
 
   bool IsContainerNode() const
   {
-    return IsElement() || !IsCharacterData();
+    return IsElement() || IsDocument() || IsDocumentFragment();
   }
 
   bool IsSlotable() const