Apple doesn't support GL_KHR_robust_buffer_access_behavior, so we don't
know if we can trust the driver to be safe with out of bounds array
accesses. It's not currently failing the out of bounds access tests, but
this may be coincidental. In general it makes sense to always clamp
array accesses when GL_KHR_robust_buffer_access_behavior is not
supported.

The validating command decoder actually enables this workaround
unconditionally, but that seems unnecessary.

Bug: angleproject:2978
Change-Id: Ib061dd4db10e044ced0dff95d0aac6a91f15b100
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2018182Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: James Darpinian <jdarpinian@chromium.org>