This reverts commit 58940f67.

Reason for revert: pixel test failures have been triaged in Gold. Relanding.

Original change's description:
> Revert "Make Mac SystemInfo reflect the currently active GPU on dual GPU machines"
> 
> This reverts commit 720a8bab.
> 
> Reason for revert: pixel test failures per http://crbug.com/984780#c13
> 
> Original change's description:
> > Make Mac SystemInfo reflect the currently active GPU on dual GPU machines
> > 
> > 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>
> 
> TBR=zmo@chromium.org,geofflang@chromium.org,kbr@chromium.org,dakshidnani@google.com
> 
> Change-Id: Iacf4b8433ac1aa4d30c51f08ddfc6489af440cc4
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: 985486, 984780, angleproject:3701
> Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1757373
> Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
> Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>

Bug: 985486, 984780, angleproject:3701
Change-Id: I8f2f26f68329a2826a92742c46471b8c092b36ed
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1765963Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
(cherry picked from commit 5ec04880)
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1775272Reviewed-by: Zhenyao Mo <zmo@chromium.org>