sync: Add pre-commit update avoidance mode + flag

sync/engine/syncer_unittest.cc

 // Copyright 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.
 //
 // Syncer unit tests. Unfortunately a lot of these tests
 // are outdated and need to be reworked and updated.
 
 #include <algorithm>
 #include <limits>
 #include <list>
@@ skipping 221 matching lines @@
 
     GetModelSafeRoutingInfo(&routing_info);
     GetWorkers(&workers);
 
     context_.reset(
         new SyncSessionContext(
             mock_server_.get(), directory(), workers,
             &extensions_activity_monitor_,
             listeners, NULL, &traffic_recorder_,
             true,  // enable keystore encryption
+            false,  // force enable pre-commit GU avoidance experiment
             "fake_invalidator_client_id"));
     context_->set_routing_info(routing_info);
     syncer_ = new Syncer();
 
     syncable::ReadTransaction trans(FROM_HERE, directory());
     syncable::Directory::Metahandles children;
     directory()->GetChildHandlesById(&trans, trans.root_id(), &children);
     ASSERT_EQ(0u, children.size());
     saw_syncer_event_ = false;
     root_id_ = TestIdFactory::root();
@@ skipping 4566 matching lines @@
     EXPECT_EQ("xyz", final_monitor_records["xyz"].extension_id);
     EXPECT_EQ(2049U, final_monitor_records["ABC"].bookmark_write_count);
     EXPECT_EQ(4U,    final_monitor_records["xyz"].bookmark_write_count);
   } else {
     EXPECT_TRUE(final_monitor_records.empty())
         << "Should not restore records after successful bookmark commit.";
   }
 }
 
 }  // namespace syncer