Enable the encodeAndDecodeSRGBForGenerateMipmap workaround for iOS.

Bug: angleproject:5417
Change-Id: I39d9e7c371f0ee5ec5119669f3b855251691f9e2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2606657Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: James Darpinian <jdarpinian@chromium.org>