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This CL adds a new onboarding dismiss prompt by:
- Introduced a superclass |PhoneHubContentView| for all content views
  with an |OnBubbleClose| function.
- Separate the previous |OnboardingView| to |OnboardingMainView| which
  is responsible for displaying the main page of the onboarding view,
  and |OnboardingDismissPromptView| which is responsible for displaying
  the dismiss prompt.
- Refactors the content view classes to inherit from the superclass.
- Updates and refactors the unittest.

Screenshot: https://screenshot.googleplex.com/7vqNqrD9im5vvVS.png

Future work:
Update the UI to spec by removing the phone status bar.

BUG=1106937,1126208

Change-Id: I6d4f164526d37e7018aa696324e8b1f7878492bd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2474253
Commit-Queue: Meilin Wang <meilinw@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Tim Song <tengs@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820325}