Bug 1343937 - Fix a crash in nsWrapperCache.h

* Implement and use GetInFlowParent
* Exempt scrollbar NAC from the new NAC semantics

Tag #1375

dom/base/nsIContent.h

@@ -950,6 +950,14 @@ public:
     return false;
   }
 
+  // Returns true if this element is native-anonymous scrollbar content.
+  bool IsNativeScrollbarContent() const {
+    return IsNativeAnonymous() &&
+           IsAnyOfXULElements(nsGkAtoms::scrollbar,
+                              nsGkAtoms::resizer,
+                              nsGkAtoms::scrollcorner);
+  }
+
   // Overloaded from nsINode
   virtual already_AddRefed<nsIURI> GetBaseURI(bool aTryUseXHRDocBaseURI = false) const override;