MFH: r466270

www/firefox: use SkiaGL by default for OpenGL/WebRender compositing

Copy OS X behavior. If OpenGL compositing[1] doesn't crash OS or browser
SkiaGL canvas rendering is unlikely to make it worse. Mainly improves
FishIE Tank benchmark.

[1] layers.acceleration.force-enabled -> true in about:config or

$ env MOZ_ACCELERATED=1 firefox -new-instance -profile `mktemp -d` ...

Approved by: ports-secteam blanket