sync: Add pre-commit update avoidance mode + flag

sync/engine/sync_scheduler_unittest.cc

 // 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.
 
 #include "base/bind.h"
 #include "base/callback.h"
 #include "base/compiler_specific.h"
 #include "base/memory/weak_ptr.h"
 #include "base/message_loop.h"
 #include "base/test/test_timeouts.h"
@@ skipping 132 matching lines @@
       workers.push_back(it->get());
     }
 
     connection_.reset(new MockConnectionManager(directory()));
     connection_->SetServerReachable();
     context_.reset(new SyncSessionContext(
             connection_.get(), directory(), workers,
             &extensions_activity_monitor_,
             std::vector<SyncEngineEventListener*>(), NULL, NULL,
             true,  // enable keystore encryption
+            false,  // force enable pre-commit GU avoidance experiment
             "fake_invalidator_client_id"));
     context_->set_routing_info(routing_info_);
     context_->set_notifications_enabled(true);
     context_->set_account_name("Test");
     scheduler_.reset(
         new SyncSchedulerImpl("TestSyncScheduler",
             BackoffDelayProvider::FromDefaults(),
             context(),
             syncer_));
   }
@@ skipping 1116 matching lines @@
       base::Bind(&CallbackCounter::Callback, base::Unretained(&counter)));
   scheduler()->ScheduleConfiguration(params);
 
   scheduler()->OnConnectionStatusChange();
   scheduler()->OnConnectionStatusChange();
 
   PumpLoop();  // Run the nudge, that will fail and schedule a quick retry.
 }
 
 }  // namespace syncer