This reverts commit ac58e632.

Reason for revert: Revert on kbr's request: "has an uninitialized variable bug"

Original change's description:
> Define and expose EGL_ANGLE_power_preference extension.
> 
> 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>

TBR=geofflang@chromium.org,kbr@chromium.org,cwallez@chromium.org

Change-Id: Icb936d91eec70deb825585da3ff8835ca2794736
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 2813
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1575134Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Kai Ninomiya <kainino@chromium.org>