[Sync] Ensure referrer is recorded regardless of policy conversion

components/sync/protocol/session_specifics.proto

Index: components/sync/protocol/session_specifics.proto
diff --git a/components/sync/protocol/session_specifics.proto b/components/sync/protocol/session_specifics.proto
index 622309b5a7e7cd7097bf3fb25739810f8a461e9b..72b7e02f2c2bdedf23cc49803113eb6515e8b800 100644
--- a/components/sync/protocol/session_specifics.proto
+++ b/components/sync/protocol/session_specifics.proto
@@ -134,8 +134,8 @@ message TabNavigation {
   // The status code from the last navigation.
   optional int32 http_status_code = 20;
 
-  // Referrer policy. Old, broken value.
-  optional int32 obsolete_referrer_policy = 21 [default = 1];
+  // Referrer policy. Old, broken value. Deprecated in M61.
+  optional int32 obsolete_referrer_policy = 21 [deprecated = true];
   // True if created from restored navigation entry that hasn't been loaded.
   optional bool is_restored = 22;
   // The chain of redirections for this navigation, from the original URL