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In mustash we need to have some D-Bus clients in the ash window manager
process and some in the browser process. In traditional ash we need everything
in the browser process. Allow per-process initialization of subsets of clients.

* Split ownership of clients into DBusClientsBrowser, DBusClientsAsh and
DBusClientsCommon
* Make DBusThreadManager::Initialize() take a process enum. This isn't great,
but see code review comments
* Remove unnecessary setters from DBusThreadManagerSetter
* Use ash/DEPS to restrict which clients can be used in //ash
* Only initialize ash clients in ash_unittests
* When running in mustash, limit which clients are initialized in the ash
and browser processes

This is a transitional step toward cleaner multi-process initialization of
clients as discussed in go/chromeos-dbus-clients option (G)

BUG=644414,647367
TEST=chromeos_unittests, ash_unittests

Review-Url: https://codereview.chromium.org/2343993003
Cr-Commit-Position: refs/heads/master@{#419481}