[Sync] Update SyncBackendHost to forward ClearServerData to SyncManager

sync/internal_api/test/fake_sync_manager.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 "sync/internal_api/public/test/fake_sync_manager.h"
 
 #include <cstddef>
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
@@ skipping 239 matching lines @@
 }
 
 ModelTypeSet FakeSyncManager::GetLastRefreshRequestTypes() {
   return last_refresh_request_types_;
 }
 
 void FakeSyncManager::SetInvalidatorEnabled(bool invalidator_enabled) {
   // Do nothing.
 }
 
+void FakeSyncManager::ClearServerData(const ClearServerDataCallback& callback) {
+  callback.Run();
+}
+
 }  // namespace syncer