This will set up the GN args for the perf builders.

Bug: angleproject:6090
Change-Id: I3c00a5b78932ae41199953a46c525b435b566d12
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2989669Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>