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This CL passes a MultiDeviceSetupClient to MultideviceHandler and
utilizes it to pass data about the current multi-device host to the
settings page.

Additionally, this CL adds functionality to the TestWebUI class which
allows it to store and respond to registered callbacks in WebUI.

A follow-up CL will add functionality which provides the status of the
various multi-device features (e.g. Instant Tethering) to the settings
page as well.

Bug: 824568
Change-Id: I5665e225122e7a2875150f61cef77aaf770298d9
Reviewed-on: https://chromium-review.googlesource.com/1157268Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580013}