Previously artifacts were not reset for a new trace in a batch

Bug: angleproject:5966
Change-Id: Ia6ed8c070eaaeca7e6cea887419666f630648df8
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2925218Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>