To enable executing code in a context of a particular time or date (e.g. when
codepath depends on whether it's say evening or New Year) there is a need for
a way to provide it bypassing actual system time.

Bug: chromium:751993
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: Iee35d97b74345f63fff814a65a6f134d7c970341
Reviewed-on: https://chromium-review.googlesource.com/598666
Commit-Queue: Sergei Datsenko <dats@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47700}