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Creates a new protocol to expose MainController internals to
SceneController.
Makes a bunch of ivars in MainController properties.

Bug: 1012697
Change-Id: Ia1607c7ed75cb0b2edf99252d9c0342237e92b26
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1848695
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707353}