[Sync] Migrate SyncInternalsMessageHandler off CallJavascriptFunctionUnsafe.

components/sync/driver/fake_sync_service.cc

 // Copyright 2015 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 "components/sync/driver/fake_sync_service.h"
 
 #include "base/memory/ptr_util.h"
 #include "base/values.h"
 #include "components/sync/driver/data_type_controller.h"
 #include "components/sync/syncable/base_transaction.h"
@@ skipping 196 matching lines @@
 void FakeSyncService::RemoveTypeDebugInfoObserver(
     TypeDebugInfoObserver* observer) {}
 
 base::WeakPtr<JsController> FakeSyncService::GetJsController() {
   return base::WeakPtr<JsController>();
 }
 
 void FakeSyncService::GetAllNodes(
     const base::Callback<void(std::unique_ptr<base::ListValue>)>& callback) {}
 
+SigninManagerBase* FakeSyncService::signin() const {
+  return nullptr;
+}
+
 }  // namespace syncer