sync: Add pre-commit update avoidance mode + flag

sync/internal_api/public/util/experiments.h

 // Copyright (c) 2012 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.
 
 #ifndef SYNC_UTIL_EXPERIMENTS_
 #define SYNC_UTIL_EXPERIMENTS_
 
 #include "sync/internal_api/public/base/model_type.h"
 
 namespace syncer {
 
 const char kAutofillCullingTag[] = "autofill_culling";
 const char kFaviconSyncTag[] = "favicon_sync";
+const char kPreCommitUpdateAvoidanceTag[] = "pre_commit_update_avoidance";
 
 // A structure to hold the enable status of experimental sync features.
 struct Experiments {
   Experiments() : autofill_culling(false),
                   favicon_sync_limit(200) {}
 
   bool Matches(const Experiments& rhs) {
     return (autofill_culling == rhs.autofill_culling &&
             favicon_sync_limit == rhs.favicon_sync_limit);
   }
 
   // Enable deletion of expired autofill entries (if autofill sync is enabled).
   bool autofill_culling;
 
   // The number of favicons that a client is permitted to sync.
   int favicon_sync_limit;
 };
 
 }  // namespace syncer
 
 #endif  // SYNC_UTIL_EXPERIMENTS_