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This CL replaces base::Timer with its subclasses.
As base::Timer changes its behavior subject to its construction time
flags, that makes hard to see the actual timer behavior, especially
it's unclear when a timer is injected from other components.
Also, that OnceCallback support of base::Timer is hard to implement on
the dynamically determined invocation pattern.

Bug: 850247
Change-Id: Ic99f2456dbb1b477ebb8f320b6a472d5a0e4c6bf
Reviewed-on: https://chromium-review.googlesource.com/1128663
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573598}