On some (mostly older, integrated) GPUs, the normal GPU texture cache
update path doesn't work well when running on ANGLE, causing CPU stalls
inside D3D and/or the GPU driver. To reduce the number of code paths we
have active that require testing, we will enable the GPU cache scatter
update path on all devices running with ANGLE. We want a better solution
long-term, but for now this is a significant performance improvement on
HD4600 era GPUs, and shouldn't hurt performance in a noticeable way on
other systems running under ANGLE.

https://hg.mozilla.org/mozilla-central/rev/0415da2e9198