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Adds a startup sequence for when the flag is enabled.
Many features are disabled, but it allows booting up and even
opening a second window.

Details:
* uses scene notifications to connect main+scene controllers
* uses scene notifications for app delegate level plumbing
* fixes window creation w/ flag set

Change-Id: I3f85f430ca916e9a866477582e646ca5b92ce4df
Bug: 1045547
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2097990
Commit-Queue: Mark Cogan <marq@chromium.org>
Auto-Submit: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#750638}