GitLab

- Move decision to show and close prompt to VrShell since it is a state
  change and shout not be part of UiSceneManager.
- Allow different content messages on the exit prompt.

Change-Id: I554948345638428fc48b763834619c40934498d9
Reviewed-on: https://chromium-review.googlesource.com/577674Reviewed-by: Christopher Grant <cjgrant@chromium.org>
Reviewed-by: Yash Malik <ymalik@chromium.org>
Commit-Queue: Christopher Grant <cjgrant@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488309}