Without DXGI, EnumDisplayDevices is the only way to get the
primary device. Previously the code would work on most configs
with a combination of AMD, Intel and NVIDIA GPUs but would fail
on more esoteric system. Like our try bots that have Matrox GPUs.

BUG=angleproject:1874
BUG=angleproject:2137

Change-Id: Ie2dfbb559001ccad2fd5b8a8fd6436e0fba9d003
Reviewed-on: https://chromium-review.googlesource.com/651629Reviewed-by: Geoff Lang <geofflang@chromium.org>