[chrome.dial] [WIP] Adds an API to get the active network ID.

Adds an onNetworkIdChanged event handler to chrome.dial that informs callers when there is a change to the set of networks being used for local discovery.

BUG=

[mark a. foltz, 2016-12-27 23:17:25]

Description was changed from

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[WIP] Change to add a network id event to chrome.dial.

BUG=
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to

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Adds an onNetworkIdChanged event handler to chrome.dial that informs callers
when there is a change to the set of networks being used for local discovery.

BUG=
==========

[mark a. foltz, 2016-12-27 23:19:56]

mfoltz@chromium.org changed reviewers:
+ btolsch@chromium.org, imcheng@chromium.org, jrw@chromium.org,
zhaobin@chromium.org

[mark a. foltz, 2016-12-27 23:19:56]

PTAL at the overall design and provide any feedback before implementation
proceeds.

The implementation will vary by platform; there isn't a single cross-platform
place to get all the necessary information.

[mark a. foltz, 2017-05-04 23:35:16]

This has been taken over here: https://codereview.chromium.org/2750453002