sync: Add pre-commit update avoidance mode + flag

sync/sessions/nudge_tracker.h

 // Copyright 2013 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.
 //
 // A class to track the outstanding work required to bring the client back into
 // sync with the server.
 #ifndef SYNC_SESSIONS_NUDGE_TRACKER_H_
 #define SYNC_SESSIONS_NUDGE_TRACKER_H_
 
 #include <list>
@@ skipping 16 matching lines @@
   static size_t kDefaultMaxPayloadsPerType;
 
   NudgeTracker();
   ~NudgeTracker();
 
   // Returns true if there is a good reason for performing a sync cycle.
   // This does not take into account whether or not this is a good *time* to
   // 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();
 
   // Takes note of a local change.
   void RecordLocalChange(ModelTypeSet types);
 
   // Takes note of a locally issued request to refresh a data type.
   void RecordLocalRefreshRequest(ModelTypeSet types);
 
@@ skipping 83 matching lines @@
 
   size_t num_payloads_per_type_;
 
   DISALLOW_COPY_AND_ASSIGN(NudgeTracker);
 };
 
 }  // namespace sessions
 }  // namespace syncer
 
 #endif  // SYNC_SESSIONS_NUDGE_TRACKER_H_