add browser integration tests

[bcoe]

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.

[JustinBeckwith]

We have a fairly large set of tools we use to do this in other libs, like https://github.com/googleapis/gaxios. I kinda wonder out loud if there's a "standardjs of webpack" that we could use or make to reduce the amount of stuff we need for this (thinking out loud)