It's not currently exposed on iOS.

Bug: angleproject:5417
Change-Id: I2564d3363f8f8bddad4da7d8a9d7dac9bea15897
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2600454Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: James Darpinian <jdarpinian@chromium.org>