downloader: extract GetCacheStats
downloader: extract GetCacheStats

I will use GetCacheStats in `isolated download` subcommand.

Bug: 932396
Change-Id: I91e01c95d0054a39b5913b39d85db3b52a31d366
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/1924346
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go