[Sync] Ensure referrer is recorded regardless of policy conversion

components/sync/protocol/session_specifics.proto

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 //
 // Sync protocol datatype extension for sessions.
 
 // Update proto_value_conversions{.h,.cc,_unittest.cc} if you change
 // any fields in this file.
 
 syntax = "proto2";
@@ skipping 116 matching lines @@
     STATE_ALLOWED = 1;
     STATE_BLOCKED = 2;
   }
   // Whether access to the URL was allowed or blocked.
   optional BlockedState blocked_state = 18 [default = STATE_ALLOWED];
   // A list of category identifiers for the URL.
   repeated string content_pack_categories = 19;
   // 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
   // through the last URL that redirected.
   repeated NavigationRedirect navigation_redirect = 23;
   // Normally not present.
   // The last URL traversed when different from the virtual_url.
   optional string last_navigation_redirect_url = 24;
   // Correct referrer policy. Valid enums are defined in
   // third_party/WebKit/public/platform/WebReferrerPolicy.h.
@@ skipping 18 matching lines @@
   // B is got by clicking a link on page of A. This relationship is used to
   // define a Chrome Task as a tree rooted by a navigation.
   repeated int64 ancestor_task_id = 28;
 }
 
 // Navigation information for a single redirection within a single navigation.
 message NavigationRedirect {
   // A URL that redirected while navigating to the virtual_url.
   optional string url = 1;
 }