Window area captured for filmstrip is too large when Chrome is launched with --enable-automation

#### Provide the steps to reproduce

1. Run [Lighthouse with Puppeteer](https://github.com/GoogleChrome/lighthouse/blob/master/docs/puppeteer.md)
2. Save the LHR
3. View it in [the Lighthouse Viewer](https://googlechrome.github.io/lighthouse/viewer/)

#### What is the current behavior?

The filmstrip screenshots capture the whole window:


#### What is the expected behavior?

Only the viewport area should be captured. I looked into why it doesn't work with Puppeteer, and found that it's fine without the `--enable-automation` flag [that Puppeteer adds](https://github.com/GoogleChrome/puppeteer/blob/0e0a67916d35672dc61580e83abf464afce10453/lib/Launcher.js#L60) when launching Chrome.


This is probably an issue with Chrome, though when I captured a filmstrip manually in DevTools it looked fine.

A workaround is passing `ignoreDefaultArgs: true` to Puppeteer and then passing in the [same args as chrome-launcher does](https://github.com/GoogleChrome/chrome-launcher/blob/master/src/flags.ts).