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This CL changes the ownership of base::Clock and base::TickClock from
injectee-owned to injecter-owned. Before this CL, these instances are
owned by the owner of the injectee or one of the injectees themselves.
That makes the ownership complex.

After this CL, the clock instances are always un-owned by injectees.
Instead, an injecter owned one is used on testing, and the shared
instance is used on production path.

Bug: 789079
Change-Id: Iff3360eb8b4d08934882cd84afc8d4d0626b456e
Reviewed-on: https://chromium-review.googlesource.com/795403Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520234}