Improve documentation on headless browser testing

[cotsog]

See #19846.

The customer suggested we improve the documentation about setuping xvfb:

http://docs.travis-ci.com/user/gui-and-headless-browsers/#Using-xvfb-to-Run-Tests-That-Require-GUI-(e.g.-a-Web-browser)

[MariadeAnton]

Hey @plaindocs!
Actually this one refers to http://docs.travis-ci.com/user/gui-and-headless-browsers/#Starting-a-Web-Server

I think user was confused because this part is also needed:

before_script:
  - "export DISPLAY=:99.0"
  - "sh -e /etc/init.d/xvfb start"
  - sleep 3 # give xvfb some time to start

to run headless tests without starting a web server after that.

[plaindocs]

Yes, I see the problem. Thanks.

[plaindocs]

@cotsog @MariadeAnton does either of you have a simple example using Firefox but not using a webserver? I think that might be a good thing to add.

Other feedback welcome.

[MariadeAnton]

I do not have a simple example using Firefox but not using a webserver. @cotsog, do you?

[ankon]

This PR un-did #311 completely, and it is not clear why one should manually start the server, if xvfb-run works just as well -- and considerably easier, and with less races as per travis-ci/travis-ci#4387)

As this PR is still open, maybe someone involved that can see the original issue could shed a bit of light on what actually was the problem?

FWIW: I'm trying to debug issues where Xvfb reports errors when starting, but would like to avoid to switch from xvfb-run to this magic if possible.

[davehunt]

Both Chrome and Firefox now natively support headless mode. See #1538 for the documentation of Firefox, and there's already documentation for Chrome.

[MariadeAnton]

Thanks so much for the help, @davehunt! We're all set now with Firefox too 😊

For xvfb - there's a PR and review on the works at #1206 - @ankon, please feel free to take a look 👀 there and let us know what you think!