New version skips unnecessary hash check for files that are already
in the cache.

R=iannucci@chromium.org
BUG=

Change-Id: Icdbd1d171fa77cf7e353c84e82ded8eafd88c88d
Reviewed-on: https://chromium-review.googlesource.com/454241Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Vadim Shtayura <vadimsh@chromium.org>