Currently, the GetSystemInfo() function (in gpu_info_util/SystemInfo_mac.mm)
on dual GPU Macs always updates the active GPU field of the SystemInfo
instance to the first non-Intel GPU it finds. This change overrides the
activeGPUIndex field of the SystemInfo instance to reflect the current GPU
instead of the non-intel GPU.

Bug: 985486, 984780, angleproject:3701
Change-Id: Ia93f38f4a2f7728c05e99c83a940722bd3808766
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1752334
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>