This should allow better integration with buildbucket named cache
support. Since this was already parameterized it shouldn't actually
break anything; previously the directory would be deleted on every run,
now it will still be deleted on every run (but we can add a buildbucket
named cache entry for it to preserve it).

R=tandrii@chromium.org, vadimsh@chromium.org

Bug: 877775
Change-Id: I1184fb59a3ed18bfda5fcb288903201b01527096
Recipe-Nontrivial-Roll: build
Recipe-Nontrivial-Roll: infra
Reviewed-on: https://chromium-review.googlesource.com/c/1263861
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>