EnumDisplayDevicesA returns the card that's connected to the display,
but EnumAdapters return the adapter which the desktop primary is
displayed at index 0. We can use this to determine the device used for
graphics.

Also cleans up the discrepancy between platforms on finding "primary" vs
"active" GPU. Asserts that the GPU expected to run ANGLE commands is the
active GPU, and deprecates the primary GPU to be equal to the active
GPU.

Bug: angleproject:3383
Change-Id: I422fba1bbe47d85b7c09e378d559eaebf89e2625
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1584360
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>