This enables SwiftShader detection.

Bug: angleproject:3876
Change-Id: Ic4ebfb5b431771c31298af7349d0370df913e596
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1773853Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>