Add headless Chrome target

[lencioni]

Now that headless Chrome is a thing, it probably shouldn't be too difficult to add Chrome as a rendering target for Happo.

[trotzig]

Yep, the only thing different from the firefox target is that the chrome webdriver doesn't support taking a screenshot of a single element (at least not last time I checked). This is the relevant code for the happo-target-firefox package:

happo/packages/happo-target-firefox/src/runVisualDiffs.js

Line 76 in c291f5f

overlay.takeScreenshot().then((screenshot) => {

[beeing]

In case if you missed this, puppeteer will be a better option.

An example of screenshot in their test code.