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1. AppState now advertises when safe mode has completed.
2. SceneControllers now don't do anything while Safe Mode is active.
3. AppState now uses a foreground scene to display safe mode.
4. MainController waits for Safe Mode to complete before any
scene-event-related setup.
5. Scenes now observe AppState, and access it directly
(and not through MainController).

Bug: 1080907
Change-Id: Ifffcd7cf7dd822d068268cc4a3709221acea2caf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2193657
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770820}