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Since we will only support the mini-container start up flow, we can
simplify the code by eliminating code paths which required to support
launching the full instance directly.

Bug: 65548422
Test: Ensure container boots and upgrades successfully.
Change-Id: Ibb1c2f7af66e3f1ea7f3495f26f34b5e47a4270a
Reviewed-on: https://chromium-review.googlesource.com/821650
Commit-Queue: Christopher Morin <cmtm@google.com>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#531818}