[Sync] Add support for retrying a getupdates due to a context change

sync/engine/update_handler.h

Index: sync/engine/update_handler.h
diff --git a/sync/engine/update_handler.h b/sync/engine/update_handler.h
index 29bdbc9deea2093f2e4ca6fd53d6bec03e3b28a4..7adfb83d12b2326a8f0123633f8f6120f68e4ba5 100644
--- a/sync/engine/update_handler.h
+++ b/sync/engine/update_handler.h
@@ -49,7 +49,10 @@ class SYNC_EXPORT_PRIVATE UpdateHandler {
   //
   // In this context, "applicable_updates" means the set of updates belonging to
   // this type.
-  virtual void ProcessGetUpdatesResponse(
+  //
+  // Returns true if the response was successfully handled, false if an error
+  // occurred and the GetUpdates should be retried.
+  virtual bool ProcessGetUpdatesResponse(
       const sync_pb::DataTypeProgressMarker& progress_marker,
       const sync_pb::DataTypeContext& mutated_context,
       const SyncEntityList& applicable_updates,