Apparently Mozilla completely spaced out on this when adding the CSP nonce
mechanism, potentially causing web compat issues if scripting relies on
being able to get/set nonces through DOM and/or causing CSP policy check
issues if websites try to "hide" nonces.
The removal of this preference was botched, all other surrounding plumbing changes appear to be working okay. The ability to use prefs to control this stylesheet might be useful in the future, so perhaps this is one of those "if it ain't broke, don't fix it" bugs where leaving well enough alone in the first place would have been the best choice.
This PR passes all current tests for this feature, and implements the "prefers-color-scheme" media query as an enumerated keyword that is controlled by an integer preference.
Currently, the possible options are 0 to see a website's fallback code and essentially behave like this isn't implemented (our current behavior), 1 to express a preference for a light theme (the default for spec reasons), and 2 to express a preference for a dark theme. Over time, this list may expand to include other preferences like a preference for a sepia theme or something, and this leaves us prepared for that future.
This is probably the last thing we will ship (if ever) since it needs the most
spec and implementation work for arbitrary use that is pretty far into a corner.
This feature should not be shipped until the various definitions of addition for
each additive property are properly specified and then implemented accordingly.
Unlike other patches in this series, compositing is not frequently used
internally so there is no need to enable this by default for chrome callers.
This preference controls whether authors are allowed to specify animations
without a 0% or 100% keyframe.
We intend to ship this but it isn't implemented yet (needs a follow-up) but this
preference acts as a safeguard in case we discover we need to disable it once
it's implemented.
ScrollbarStyles contains values of overflow, (over)scroll-behavior, etc.
The only one which is marginally related to scroll _bars_ is overflow, which can
be used to hide scrollbar (by making an element not scrollable) or enforce the
scrollbar to display.
It makes more sense to be called ScrollStyles as it's mainly concerning behavior
of scrolling, not scrollbars. Also, with the addition of scrollbar width
properties, the current name can be confusing.
