[network service] Add a NetworkChangeManager interface

This CL adds an interface to allow consumers to subscribe/unsubscribe
to network change events.

- mojom::NetworkChangeManager
  Listens to net::NetworkChangeNotifier and propagate notifications to
  mojom::NetworkChangeManagerClient.
  NetworkService will have a pointer to the impl of this interface.

- mojom::NetworkChangeManagerClient
  The implementation of this interface(content::NetworkConnectionTracker)
  receives notifications from mojom::NetworkChangeManager, and then sends
  those down to its observers.

Design doc:
https://docs.google.com/document/d/1kBp_vTIH-1Jx4M9DN7mnqRRyUmeHe3BZ1NqTbsJ4_S8/edit

Bug: 754709
Change-Id: Ia98dc41da9bb618fd00adfd6412d70d839cd13bb
Reviewed-on: https://chromium-review.googlesource.com/644352
Commit-Queue: Helen Li <xunjieli@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Paul Jensen <pauljensen@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#510117}

• content/network/BUILD.gn[diff]
• content/network/DEPS[diff]
• content/network/network_change_manager_impl.cc[Added - diff]
• content/network/network_change_manager_impl.h[Added - diff]
• content/network/network_change_manager_impl_unittest.cc[Added - diff]
• content/network/network_service_impl.cc[diff]
• content/network/network_service_impl.h[diff]
• content/network/network_service_unittest.cc[diff]
• content/public/common/BUILD.gn[diff]
• content/public/common/DEPS[diff]
• content/public/common/network_change_manager.mojom[Added - diff]
• content/public/common/network_connection_tracker.cc[Added - diff]
• content/public/common/network_connection_tracker.h[Added - diff]
• content/public/common/network_connection_tracker_unittest.cc[Added - diff]
• content/public/common/network_service.mojom[diff]
• content/test/BUILD.gn[diff]
• net/BUILD.gn[diff]