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

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

Index: chrome/browser/sync/test/integration/two_client_uss_sync_test.cc
diff --git a/chrome/browser/sync/test/integration/two_client_uss_sync_test.cc b/chrome/browser/sync/test/integration/two_client_uss_sync_test.cc
index c1a597d01587ba3402366ebfcbbe4a96d806e053..8a6f76c07573cb945ee6fe683cb90f1595bf8e12 100644
--- a/chrome/browser/sync/test/integration/two_client_uss_sync_test.cc
+++ b/chrome/browser/sync/test/integration/two_client_uss_sync_test.cc
@@ -303,6 +303,9 @@ IN_PROC_BROWSER_TEST_F(TwoClientUssSyncTest, ConflictResolution) {
 
   // 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