Enable robustnessEXT for SwiftShader. Bug: chromium:1287631 Bug: angleproject:6906 Change-Id: I3c8c32561a1ac7860c83b8d3ba22aafd0bf81fd4 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3390489 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Roman Lavrov <romanl@google.com>