Allows application to select the integrated or discrete GPU on
dual-GPU macOS systems.

Tested by modifying the example program at:
https://github.com/grorg/ANGLEIOSurfaceTest

and verifying that both integrated and discrete GPUs can be selected.
(The changes to that program will be upstreamed once some build issues
are resolved.)

Bug: 2813
Change-Id: Ibeb17778512896456d220e9bc4cf8f222aa57057
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1570081
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>