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This CL adds the notification opt in view on the Phone Hub panel to
provide another entry point for user to grant access for notifications
from the phone. This CL also introduces a |PhoneHubViewID| enum for
easy access to main components of the bubble view during unittests.

Screenshot: https://screenshot.googleplex.com/3XBngS9RArKjD44.png

BUG=1106937,1126208

Change-Id: Ie440ebad2080382f48e48778eb94f559474aa5ed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2424647
Commit-Queue: Meilin Wang <meilinw@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810071}