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This change also factors out TestTimerFactory to reuse code.

R=khorimoto@chromium.org

Bug: none
Test: DisconnectTetheringOperationTest.*
Change-Id: I5988d6bfdbf145511d9ea5c7451c31b9d929b753
Reviewed-on: https://chromium-review.googlesource.com/c/1357614
Commit-Queue: James Hawkins <jhawkins@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613187}