Use base::Time for modification time instead of a plain int64_t

Also, update the comment about the 'modification_time' field in the
compression request.

BUG=889703

Change-Id: I37df9e8abc50b195b73ffd9cf101444dc682ff33
Reviewed-on: https://chromium-review.googlesource.com/c/1279199
Commit-Queue: Anand Mistry <amistry@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599735}
6 files changed