import from UXP: [accessibility] Hold a strong ref to mParent in AccIterator. (b2dc9706)

accessible/base/AccIterator.h

@@ -56,7 +56,7 @@ private:
   {
     explicit IteratorState(Accessible* aParent, IteratorState* mParentState = nullptr);
 
-    Accessible* mParent;
+    RefPtr<Accessible> mParent;
     int32_t mIndex;
     IteratorState* mParentState;
   };