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Additionally, add a few fixes which were needed to make tests pass now
that Phone Hub is active by default:
(1) Call SystemTray->SetPhoneHubManager(nullptr) when
    PhoneHubManagerFactory is deleted.
(2) Set service to NULL while testing.
(3) Do not create the service when the Profile is created; instead,
    Create it in UserSessionInitializer.
(4) Update PhoneHubNotificationController to behave correctly when a
    null PhoneHubManager is passed to it.
(5) Do not assume that there can only be 0 or 1 Tether networks. Though
    this is true in practice, there are tests which use multiple
    networks.

Fixed: 1106937
Change-Id: I15d02e114fc81284cdad072c25c3dca24ff5bd62
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2626103
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Regan Hsu <hsuregan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843761}