Replace PhantomJS by Headless Chrome

[hgwood]

Supersedes #168.

Chrome 59 will support headless mode with pdf printing.

See branch path-to-headless-chrome and #187.

Questions & answers:

• should we require Chrome to be installed on the host or should we use a docker image?
  • both
• can CircleCI handle a docker run in the build?
  • yes, but turns out no need (thanks to puppeteer)
• should we build a custom image?
  • no (thanks to puppeteer) Progress:
• [X] Migrate PDF generation for slides to Chrome (#192)
• [X] Refactor to use Chrome Launcher (see here) Puppeteer (#201)
• [X] Build a new Docker image that is capable of generating the PDFs with Headless Chrome (#201)

PDF generation for slides is done (#187). Need to do the same for labs.

• [X] Replace PhantomJS by Chrome for labs PDF printing (#202)

[jlandure]

We could use both: host install and docker image. For CircleCI, it supports a docker run. We could use the official image if exists or build our own image with all the dependencies. Sample circle.yml with docker run commands: https://github.com/Zenika/POC_EasyToGo/blob/master/circle.yml

[hgwood]

Progress today:

• this docker image works well; pdf printing requires --entrypoint (see second example here)
• when pointed to the html slides, the pdf only has a white page with the "back-to-top" arrow in the lower left corner, because the slides are not loaded yet (this is async)
• to have the slides displayed in the pdf, a script like this one that we have for phantom must be written for headless chrome
• this script can use chrome-remote-interface like shown here
• but pdf printing requires remote debugger to be disabled? there is an error message if --print-pdf --remote-debugging-port=... are specified
• so how to drive the browser?

need to investigate more:

• will pdf printing and remote debugger be able to both work at the same time in near future release?
• could we pre-render slides instead so that no JS is involved and no driving would be required

code is on branch headless-chrome-experiments.

[hgwood]

According to this, PDF printing can be triggered via the remote interface without --print-pdf being there, and that's actually the preferred method.

[hgwood]

PR: #192.