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Creates a new superclass for scene agents.
Existing scene agents are all scene state observers, and all have a
sceneState property.
It also seems useful to have a way to retrieve a scene agent for a
given scene.

These use features are factored out in a lightweight ObservingSceneAgent
class. Existing agents are made subclasses of that.

Bug: none
Change-Id: Ia1908e339178e52396294fa4f308d759ee32a6ed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2568271
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833337}