This reverts commit 2762641a.

Reason for revert: is_apple is not universally defined. Breaks ANGLE roll into Skia.

Original change's description:
> Use is_apple instead of is_mac and is_ios everywhere
>
> Bug: chromium:1161513
> Change-Id: Id7d582f127f4b79ea6fc5b8ad39fff768c45f477
> Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2611312
> Reviewed-by: James Darpinian <jdarpinian@chromium.org>
> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>

TBR=jdarpinian@chromium.org,syoussefi@chromium.org

Bug: chromium:1161513
Change-Id: I831e60149f410044a79d9f66aa9ddd9902410fd7
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2618039Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>