sync: Add pre-commit update avoidance mode + flag

sync/sessions/nudge_tracker.h

Index: sync/sessions/nudge_tracker.h
diff --git a/sync/sessions/nudge_tracker.h b/sync/sessions/nudge_tracker.h
index 0e8ab51202c058b8f72d115e5f21d5eba7c174de..f8109a42d38e7460920a4d1a7017cf75f989ae15 100644
--- a/sync/sessions/nudge_tracker.h
+++ b/sync/sessions/nudge_tracker.h
@@ -34,6 +34,10 @@ class SYNC_EXPORT_PRIVATE NudgeTracker {
   // perform a sync cycle; that's the scheduler's job.
   bool IsSyncRequired() const;
 
+  // Returns true if there is a good reason for performing a get updates
+  // request as part of the next sync cycle.
+  bool IsGetUpdatesRequired() const;
+
   // Tells this class that all required update fetching and committing has
   // completed successfully.
   void RecordSuccessfulSyncCycle();