diff --git a/chrome/browser/sharing/sharing_message_sender.h b/chrome/browser/sharing/sharing_message_sender.h
index 529ff73d..999c4d0 100644
--- a/chrome/browser/sharing/sharing_message_sender.h
+++ b/chrome/browser/sharing/sharing_message_sender.h
@@ -112,6 +112,10 @@
   std::map<std::string, SentMessageMetadata> message_metadata_;
   // Map of FCM message_id to random GUID.
   std::map<std::string, std::string> message_guids_;
+  // Map of FCM message_id to received ACK response messages.
+  std::map<std::string,
+           std::unique_ptr<chrome_browser_sharing::ResponseMessage>>
+      cached_ack_response_messages_;
 
   // Registered delegates to send messages.
   std::map<DelegateType, std::unique_ptr<SendMessageDelegate>> send_delegates_;