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This cl introduces the basic architecture to support the borealis
starter flow. It introduces the borealis context manager, which is
intended to manage the start up flow of the Borealis VM and be the
source of truth for run-time information about the Borealis VM. See
go/borealis-starter-flow for additional context.

Tests: Wrote and ran tests
Bug: b:168425531
Change-Id: I88bece091d5d9a5af4a121ff498c2ebdc504cff1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2434020
Commit-Queue: Daniel Ng <danielng@google.com>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Nic Hollingum <hollingum@google.com>
Cr-Commit-Position: refs/heads/master@{#814512}