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The operation ("what time is it?") is conceptually const, so make the
method so. Implementations that mutate their state now have such state
expliclty marked as mutable.

This change was inspired by review comments on product code that uses a
TickClock for the sake of testability, in which the reviewers wondered
why a TickClock* class member was not const. Prior to this change, the
answer was "because it can't be". Following this change, the answer can
be "because I forgot to do that. Thanks!"

BUG=none

Change-Id: I813c5359b143c52d86306200ef50eeac4a0003d1
Reviewed-on: https://chromium-review.googlesource.com/980052Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545944}