Trace Tests: Clean up desktop skips. These were confusing SwiftShader and ANGLE / Native drivers. Fix these by using helper variables. This isn't an issue on Android because our Android tests don't have SwiftShader. Bug: angleproject:7887 Change-Id: I414f59a8eb1155aaf8b98bf5c3104273cbb5b72d Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4178015 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>