[sync] Reland of "TwoClientBookmarksSyncTest.Sanity" is flaky

chrome/browser/sync/test/integration/two_client_uss_sync_test.cc

 // Copyright 2016 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/macros.h"
 #include "base/memory/ptr_util.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "chrome/browser/sync/chrome_sync_client.h"
 #include "chrome/browser/sync/profile_sync_service_factory.h"
 #include "chrome/browser/sync/test/integration/profile_sync_service_harness.h"
@@ skipping 285 matching lines @@
   ASSERT_TRUE(SetupSync());
   TestModelTypeSyncBridge* model0 = GetModelTypeSyncBridge(0);
   TestModelTypeSyncBridge* model1 = GetModelTypeSyncBridge(1);
   model0->SetConflictResolution(ConflictResolution::UseNew(
       FakeModelTypeSyncBridge::GenerateEntityData(kKey1, kValue3)));
   model1->SetConflictResolution(ConflictResolution::UseNew(
       FakeModelTypeSyncBridge::GenerateEntityData(kKey1, kValue3)));
 
   // Write conflicting entities.
   model0->WriteItem(kKey1, kValue1);
+  // Wait for the server to see the first commit to avoid a race condition where
+  // both clients commit without seeing each other's update.
+  ASSERT_TRUE(ServerCountMatchStatusChecker(syncer::PREFERENCES, 1).Wait());
   model1->WriteItem(kKey1, kValue2);
 
   // Wait for them to be resolved to kResolutionValue by the custom conflict
   // resolution logic in TestModelTypeSyncBridge.
   ASSERT_TRUE(DataChecker(model0, kKey1, kValue3).Wait());
   ASSERT_TRUE(DataChecker(model1, kKey1, kValue3).Wait());
 }
 
 IN_PROC_BROWSER_TEST_F(TwoClientUssSyncTest, Error) {
   ASSERT_TRUE(SetupSync());
@@ skipping 42 matching lines @@
 
   ASSERT_TRUE(DataChecker(model1, kKey1, kValue2).Wait());
   ASSERT_TRUE(DataChecker(model1, kKey2, kValue1).Wait());
   ASSERT_TRUE(DataChecker(model1, kKey4, kValue1).Wait());
 
   ASSERT_TRUE(DataChecker(model0, kKey1, kValue2).Wait());
   ASSERT_TRUE(DataChecker(model0, kKey3, kValue1).Wait());
 }
 
 }  // namespace