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Adds a SceneState observer protocol, and makes the SceneController be
created by the SceneDelegate and listen to this protocol.

Bug: none
Change-Id: I10378f6034f13a4b47687fcbcec6a38d5c078666
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1789585
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Auto-Submit: Stepan Khapugin <stkhapugin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697979}