This is a reland of ac58e632

The original CL was reverted in a rush because I thought there was an
uninitialized variable bug, but upon later re-review this turned out to
not be the case.

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>

Bug: 2813
Tbr: geofflang@chromium.org
Tbr: cwallez@chromium.org
Change-Id: Iea000dd718f4f4b4f57237adb1dc44381b10106b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1575419Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>