add browser integration tests

We claim in various responses to issues and docs that `google-api-nodejs-client` can be used with WebPack, and deployed in a browser context.

We, however, have no integration tests that ensure that this is possible ... in fact, we've broken the ability to WebPack `google-api-nodejs-client` a few times in recent months, due to introducing Node.js-specific APIs.

We should:

- [ ] add integration tests (using karma, puppeteer, mocha) that exercise deployment in a browser environment.
- [ ] add an integration test for creating a WebPack bundle.
- [ ] add a sample that demonstrates bundling `google-api-nodejs-client` using WebPack.