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This CL adds the initial connecting UI that will show up when the device
is trying to connect with the phone when user has opted in the Phone Hub
feature. Remaining work includes implementing the button click handler.

Demo: https://screenshot.googleplex.com/BeLuL2JWNbmmZLs.png

Misc:
Refactors the |SetButtons| API to use unique_ptr.

BUG=1106937,1126208

Change-Id: I537c1da298fbaf4fb67d0df38712618e2496fb3b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2414127
Commit-Queue: Meilin Wang <meilinw@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Tim Song <tengs@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808539}