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This CL adds handlers for "Get started" and "Dismiss" buttons on the
onboarding screen.

Misc: Update unittests in |PhoneHubUiControllerTest|.

BUG=1106937,1126208
TEST=unittested.

Change-Id: I6576b3fb32c1bd5b6a96a61ff9fcdc5ac9c2fc70
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2439363
Commit-Queue: Meilin Wang <meilinw@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Tim Song <tengs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812954}