Client-side changes to support retry GU.

sync/protocol/sync_enums.proto

Index: sync/protocol/sync_enums.proto
diff --git a/sync/protocol/sync_enums.proto b/sync/protocol/sync_enums.proto
index 348d6d1d3549f9358850628c10cff606a456b7fc..90c652b594168b38ea3c2867e967ad5cfab9eeba 100644
--- a/sync/protocol/sync_enums.proto
+++ b/sync/protocol/sync_enums.proto
@@ -141,8 +141,10 @@ message SyncEnums {
                            // confused with FIRST_UPDATE.
     RECONFIGURATION = 10;  // The client is in configuration mode because the
                            // user opted to sync a different set of datatypes.
-    GU_TRIGGER = 12; // The client is in 'normal' mode.  It may have several
-                     // reasons for requesting an update.  See the per-type
-                     // GetUpdateTriggers message for more details.
+    GU_TRIGGER = 12;  // The client is in 'normal' mode.  It may have several
+                      // reasons for requesting an update.  See the per-type
+                      // GetUpdateTriggers message for more details.
+    RETRY = 13;  // A retry GU to pick up updates missed by last GU due to
+                 // replication delay, missing hints, etc.
   }
 }