Swiftshader does not currently have the ability to behave in a way
for ANGLE to support KHR_robustness. Disable for now until that
functionality is available.

Bug: angleproject:3058
Bug: swiftshader:145
Change-Id: I1c1b8147f6b9cf6f8d0da633dfe0f61ebfab5175
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2097053
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Ian Elliott <ianelliott@google.com>