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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: I38b7d932304466a9d86bbd577f9918149914f8ea
Reviewed-on: https://chromium-review.googlesource.com/1128694Reviewed-by: Martin Barbella <mbarbella@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573522}