primaryGPUIndex is being deprecated in favor of activeGPUIndex, which
represends the expected index of the GPU used for graphics.

Bug: angleproject:3383
Change-Id: I37f992adaf3716fc11e7f34b897c51c0148719cd
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1592055Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>