Review Mozilla 1517786: Implement and ship CanvasRenderingContext2D.getContextAttributes()

Bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1517786

Might expose interesting info.

From 68ad20d8, color space and desynchronized aren't exposed (I'm particularly worried about gamut and HDR displays), but there are another couple of properties that we should understand how are initialized, and maybe add new lies.