This is also needed for listening to prefs changes in order to enable/disable Tether when 'TetherAllowed' changes. will also expose if Tether is enabled/disabled to settings. Bumping up priority.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/8d01f3ac9dad3f4833d868e253989ed88b705d1c

commit 8d01f3ac9dad3f4833d868e253989ed88b705d1c
Author: hansberry <hansberry@chromium.org>
Date: Mon Apr 24 18:58:46 2017

Create TetherService, a TetherAllowed pref, and use DependencyManager for RegisterProfilePrefs calls.

TetherService dynamically starts and stops Tether when prefs::kTetherAllowed changes value (from policy changing). This is expected from policy settings in Chrome. Please see: https://sites.google.com/a/chromium.org/dev/developers/how-tos/enterprise/adding-new-policies.

This CL also tweaks EasyUnlockServiceFactory and ChromeCryptAuthServiceFactory to call RegisterProfilePrefs on their respective services. These calls are automagically handled by DependencyManager: it ensures that the RegisterProfilePrefs calls are made in the correct order and only once per context.

A subsequent CL will come after this one that actually creates the enterprise policy for Tether, and maps to prefs::kTetherAllowed.

BUG= 710174 

Review-Url: https://codereview.chromium.org/2819713002
Cr-Commit-Position: refs/heads/master@{#466702}

[modify] https://crrev.com/8d01f3ac9dad3f4833d868e253989ed88b705d1c/chrome/browser/chromeos/BUILD.gn
[modify] https://crrev.com/8d01f3ac9dad3f4833d868e253989ed88b705d1c/chrome/browser/chromeos/login/session/user_session_manager.cc
[add] https://crrev.com/8d01f3ac9dad3f4833d868e253989ed88b705d1c/chrome/browser/chromeos/tether/BUILD.gn
[add] https://crrev.com/8d01f3ac9dad3f4833d868e253989ed88b705d1c/chrome/browser/chromeos/tether/DEPS
[add] https://crrev.com/8d01f3ac9dad3f4833d868e253989ed88b705d1c/chrome/browser/chromeos/tether/OWNERS
[add] https://crrev.com/8d01f3ac9dad3f4833d868e253989ed88b705d1c/chrome/browser/chromeos/tether/tether_service.cc
[add] https://crrev.com/8d01f3ac9dad3f4833d868e253989ed88b705d1c/chrome/browser/chromeos/tether/tether_service.h
[add] https://crrev.com/8d01f3ac9dad3f4833d868e253989ed88b705d1c/chrome/browser/chromeos/tether/tether_service_factory.cc
[add] https://crrev.com/8d01f3ac9dad3f4833d868e253989ed88b705d1c/chrome/browser/chromeos/tether/tether_service_factory.h
[modify] https://crrev.com/8d01f3ac9dad3f4833d868e253989ed88b705d1c/chrome/browser/cryptauth/chrome_cryptauth_service_factory.cc
[modify] https://crrev.com/8d01f3ac9dad3f4833d868e253989ed88b705d1c/chrome/browser/cryptauth/chrome_cryptauth_service_factory.h
[modify] https://crrev.com/8d01f3ac9dad3f4833d868e253989ed88b705d1c/chrome/browser/prefs/browser_prefs.cc
[modify] https://crrev.com/8d01f3ac9dad3f4833d868e253989ed88b705d1c/chrome/browser/profiles/chrome_browser_main_extra_parts_profiles.cc
[modify] https://crrev.com/8d01f3ac9dad3f4833d868e253989ed88b705d1c/chrome/browser/signin/easy_unlock_service_factory.cc
[modify] https://crrev.com/8d01f3ac9dad3f4833d868e253989ed88b705d1c/chrome/browser/signin/easy_unlock_service_factory.h
[modify] https://crrev.com/8d01f3ac9dad3f4833d868e253989ed88b705d1c/chrome/common/pref_names.cc
[modify] https://crrev.com/8d01f3ac9dad3f4833d868e253989ed88b705d1c/chrome/common/pref_names.h