Restore canvas fingerprinting detection to Firefox

[ghostwords]

This is the followup to #1053. We should make canvas fingerprinting detection cross-browser; src/fingerprinting.js currently relies on some Chrome-specific APIs.

[fedde-s]

Are there short-term plans to release a bugfix update that disables the functionality on non-V8 browsers to make them usable again until the code is improved, as you seem to suggest in the closed pull request you linked? I believe it is currently making various web applications unusable while PB is running, also being mentioned in #1157 and several closed issues about previous occurrences of the same breakage.

[ghostwords]

Yes, absolutely, we should do that on Firefox ASAP. I would if I could.

Update: Should be fixed with Privacy Badger version 2017.1.26.1, released a few minutes ago.

[ghostwords]

Should also take care of #1268 at the same time.

[cowlicks]

In #1505 I have a cross-platform way to get the stacktrace and origin associated with some Javascript execution. This should fix this issue. (link):

function scriptOrigin() {
  try {
    yo = dog;  // eslint-disable-line
  } catch (e) {
    return e.stack.split('\n')[3].split('/')[2];
  }
}

[ghostwords]

@cowlicks I suggest reviewing OpenWPM's implementation here: https://github.com/citp/OpenWPM/blob/a6eb64afd48fe0e2b09be0b65d243f5d64a37a18/automation/Extension/firefox/data/content.js#L183

[ghostwords]

Also our original Firefox implementation here: https://github.com/EFForg/privacybadgerfirefox-legacy/blob/00d88bbf42649d31bba181b49a7886c8381682ae/data/fingerprinting.js#L101