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This is an initial CL that enables mojo for Settings ands includes a
small isolated useage.

The network_config mojo API will be used in parallel with the
networkingPrivate extension API while the code is transitioned.

Forthcoming CLs will transition the rest of Internet Settings to use
the mojo API, eventually fully replacing the extension API.

Adding mojo bindings to the Settings page resulted in no measurable
load time regressions. See issue for details.

This CL:
* Modifies SettingsUI to derive from MojoWebUIContriller
* Binds network_config mojo service to SettingsUI and OSSettingsUI
* Implements device enable API call in internet_page using Mojo

Bug: 853953
Change-Id: I99c3d2064361abb51368acc3811c45341f2db56b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1652802
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#669033}