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This CL adds the connection error UI that pops up when the Phone Hub
feature is not available due to connection issues. Remaining work
includes implementing the button click handler.

Demo: https://screenshot.googleplex.com/6VSGgicU284Z4Ts.png

Misc: updates the comment for |PhoneHubInterstitalView|.

BUG=1106937,1126208

Change-Id: I12c05c2bf47dd017af801874170aba29aa3554c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2417296
Commit-Queue: Meilin Wang <meilinw@chromium.org>
Reviewed-by: Tim Song <tengs@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808630}