[Sync] Migrate SyncInternalsMessageHandler off CallJavascriptFunctionUnsafe.

components/sync/driver/fake_sync_service.cc

Index: components/sync/driver/fake_sync_service.cc
diff --git a/components/sync/driver/fake_sync_service.cc b/components/sync/driver/fake_sync_service.cc
index 71abee3786be6c098cfbff52b8893489e0e84a44..e0d00fd0caeafc0a267121ad14c7d2c2acc987bc 100644
--- a/components/sync/driver/fake_sync_service.cc
+++ b/components/sync/driver/fake_sync_service.cc
@@ -214,4 +214,8 @@ base::WeakPtr<JsController> FakeSyncService::GetJsController() {
 void FakeSyncService::GetAllNodes(
     const base::Callback<void(std::unique_ptr<base::ListValue>)>& callback) {}
 
+SigninManagerBase* FakeSyncService::signin() const {
+  return nullptr;
+}
+
 }  // namespace syncer