In RFP mode, canvas image extraction leads to an all-white image, replace that
with a random (sample 32 bytes of randomness and fill the buffer with that)
'poison pill'. This helps defeat naive fingerprinters by producing a random
image on every try. This feature is toggled using a new, default on, pref
`privacy.resistFingerprinting.randomDataOnCanvasExtract`.
Updated `browser_canvas_fingerprinting_resistance.js` to test this new feature
as well.
Updates and replaces D66308.
Differential Revision: https://phabricator.services.mozilla.com/D72716
