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Extend the TPM firmware update code to be able to deal with different
TPM firmware update flows, represented by a newly introduced Mode
enum. In addition to the previously-existing powerwash update flow,
add a mode constant for the state preserving update flow. Change the
chrome://chrome UI to request state-preserving TPM updates if allowed.

BUG=chromium:788719
TEST=New unit tests, manual integration testing against platform functionality.

Change-Id: I7b2340b0a470d9f4bb393cde0ca01d91db3d442e
Reviewed-on: https://chromium-review.googlesource.com/1016918
Commit-Queue: Mattias Nissler <mnissler@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561443}