Add mipmap generation perftest Bug: angleproject:4551 Change-Id: Iae205674029888447d070e0df30b65828b6fa6eb Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2246732 Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Reviewed-by: Tim Van Patten <timvp@google.com> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>