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The fallback will be enabled on Neverware devices using the added build
flag: system_slot_software_fallback. Its default value is false and this
behavior is not intended for Chromebooks which always have a TPM.

In case Neverware enables the build flag:
  (a) Devices with a TPM will still initialize a TPM-backed system slot.
  (b) Devices without a TPM will initialize a software-backed system
      slot.

DD: https://docs.google.com/document/d/1rbTiAMpR18SoN0be_AW-ysvptAHbYcTYeZHL6KcAcTQ/edit?usp=sharing
Initial bug discussion: https://docs.google.com/document/d/16w4xyCi2tQTIBtS8o4Ga01KxpHwICVnCjyGC6ezlNS0/edit?usp=sharing

Bug: b:171461831
Change-Id: I533a7de7bf5367143aa539b9a7883abd6d8f405a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2552600
Commit-Queue: Maria Petrisor <mpetrisor@chromium.org>
Reviewed-by: Mattias Nissler <mnissler@chromium.org>
Reviewed-by: Alexander Hendrich <hendrich@chromium.org>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833385}