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Introduces a temporary interface connecting the Scene and the Main
controllers, and exposes a bunch of internals of MainController through
a MainControllerGuts protocol (obviously temporary).

Uses this interface to implement ApplicationCommands in SceneController
and updates the use of the commands endpoint to be SceneController.

Note also:
- Updates a bunch of ivar use in MainController to properties.
- Moves a Deferred Initialization constant to a shared file as it's
used in both SceneCtrl and MainCtrl. It seems like this is semantically
correct for it to be used in both places, so it probably is a permanent
change.
- Removes a DCHECK(_localStatePrefObserverBridge) in
showSettingsFromViewController, as keeping it would mean exposing MainController->_localStatePrefObserverBridge to SceneController.

Bug: None
Change-Id: Idd45bc0850eaabca068e59e3843d901c9a0c780b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1845976
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709497}