sync: Add pre-commit update avoidance mode + flag

chrome/browser/sync/glue/sync_backend_host.cc

Index: chrome/browser/sync/glue/sync_backend_host.cc
diff --git a/chrome/browser/sync/glue/sync_backend_host.cc b/chrome/browser/sync/glue/sync_backend_host.cc
index d53f7cce4a94570bfb19da5b0053182fc8a3c62d..ac9e6333a757cf5143f2e961b98ec37300b00d05 100644
--- a/chrome/browser/sync/glue/sync_backend_host.cc
+++ b/chrome/browser/sync/glue/sync_backend_host.cc
@@ -383,6 +383,10 @@ void SyncBackendHost::Initialize(
     factory_switches.backoff_override =
         InternalComponentsFactoryImpl::BACKOFF_SHORT_INITIAL_RETRY_OVERRIDE;
   }
+  if (cl->HasSwitch(switches::kSyncEnableGetUpdateAvoidance)) {
+    factory_switches.pre_commit_updates_policy =
+        InternalComponentsFactoryImpl::FORCE_ENABLE_PRE_COMMIT_UPDATE_AVOIDANCE;
+  }
 
   initialization_state_ = CREATING_SYNC_MANAGER;
   InitCore(DoInitializeOptions(