Add origin trials for webgl 2

[jdm]

Our WebGL 2 implementation is not yet at the point where we can enable it by default (in particular, there are problems like #26142). However, for demos that require WebGL 2 and run correctly with it enabled like http://joshmatthews.net/webxr-particles/, we should add support for enabling it on known-good origins. We can do this by using a Func annotation instead of Pref in the various webgl2 WebIDL files, and the function can first check the preference, and then fall back to comparing the global's origin against a list of known good origins.

[jdm]

Implemented in #26191.