Compiling Electron leads to firestore breaking due to grpc issues.

Hey there! I am currently developing a desktop application using electron that is able to run as an admin client for our managers at work. I've run into an issue with using firebase-admin where when electron is in dev mode it works fine, but when compiled into an exe format it breaks and the error it has is saying that I need to "pass auth instance to grpc-fallback-client" and its coming from `new GrpcClient` looking further into the code about how this is made its a google-gax client.

Not sure if this is the correct area to put this but I've tried other areas and found nothing. The only similar issue I have seen is googleapis/google-cloud-node-core#445

#### Environment details

- OS: Windows
- Node.js version: 8.16.1
- npm version: 6.4.1
- `google-gax` version: 1.3.0

#### Steps to reproduce
(very custom way to hit this and I don't really know how to walk someone through getting to this exact error)
1. Have an Electron app that you can compile to exe
2. inside the main.js or type script file add an admin client from `firebase-admin`
3. Authenticate your admin client
4. Try and pull a collection from firestore

Thanks!

Beginne damit, die in den Schritten beschriebene paketierte Windows-Electron-App zu reproduzieren, indem du den firebase-admin Firestore-Client aus main.js oder dem TypeScript-Einstiegspunkt verwendest. Verfolge den gemeldeten Fehler ab new GrpcClient in google-gax und vergleiche die Entwicklungsausführung mit der kompilierten Ausführung. Erledigt ist die Aufgabe, wenn die kompilierte Anwendung sich authentifiziert und eine Firestore-Sammlung abruft, ohne den grpc-fallback-client-Fehler zu verursachen.

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