[Sync] Log age of auth tokens on authentication failure

sync/internal_api/public/sync_manager.h

Index: sync/internal_api/public/sync_manager.h
diff --git a/sync/internal_api/public/sync_manager.h b/sync/internal_api/public/sync_manager.h
index 2d71466d7ff4784896db0395aeb088579b12a6bd..d754bf01a35a72bef55b82eeeff573c07c98054e 100644
--- a/sync/internal_api/public/sync_manager.h
+++ b/sync/internal_api/public/sync_manager.h
@@ -61,8 +61,12 @@ enum ConnectionStatus {
 
 // Contains everything needed to talk to and identify a user account.
 struct SyncCredentials {
+  // The email associated with this account.
   std::string email;
+  // The raw authentication token's bytes.
   std::string sync_token;
+  // (optional) The time at which the token was fetched/refreshed.
+  base::Time sync_token_time;
 };
 
 // SyncManager encapsulates syncable::Directory and serves as the parent of all