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The Scene is meant to hold state supplied by the SceneManager, and
consumed by the rendering subsystem.  Scene shouldn't know about modes,
prompts, etc.  This CL tries to banish any such state.

BUG=

Change-Id: I5cab334aac5150b725ab6408625367d0554bdabf
Reviewed-on: https://chromium-review.googlesource.com/576471
Commit-Queue: Christopher Grant <cjgrant@chromium.org>
Reviewed-by: Yash Malik <ymalik@chromium.org>
Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488267}