Adds --retain option (all, none, json) which controls which files are
kept after running the benchmark.

No-Try: true
Change-Id: I033a8ff7938fd10f8a0fc9425043e24f1229ce0e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1980575Reviewed-by: Mythri Alle <mythria@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65553}