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We always end up using DBusThreadManagerLinux on Linux and
DBusThreadManager on Chrome OS. Rather than having clients pass
these when initializing BluezDBusThreadManager, have
BluezDBusThreadManager choose which one to use based on the OS.

This puts the logic for choosing which one to use in a single place.

Bug: 882771
Change-Id: I9c36eb9de8419fd5ac4bc6fbab3ded2738b33d91
Reviewed-on: https://chromium-review.googlesource.com/1237694Reviewed-by: Sonny Sasaka <sonnysasaka@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Commit-Queue: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593730}