Bug 1156006 - Mark NodeInfo::mDocument as MOZ_NON_OWNING_REF; r=baku

dom/base/NodeInfo.h

@@ -276,7 +276,9 @@ protected:
   // nsNodeInfoManager needs to pass mInner to the hash table.
   friend class ::nsNodeInfoManager;
 
-  nsIDocument* mDocument; // Weak. Cache of mOwnerManager->mDocument
+  // This is a non-owning reference, but it's safe since it's set to nullptr
+  // by nsNodeInfoManager::DropDocumentReference when the document is destroyed.
+  nsIDocument* MOZ_NON_OWNING_REF mDocument; // Cache of mOwnerManager->mDocument
 
   NodeInfoInner mInner;