Add option to disable SwiftShader when retracing. Bug: angleproject:5133 Change-Id: I589252d7eb9648a37a98f896c2a14462b2b34c48 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2506492 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Cody Northrop <cnorthrop@google.com> Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>