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In order to make a better unittest for this function and to re-use the
function in the rewritten attestation flow in the future, the mentioned
function is moved out as a stand-alone function.

BUG=b:158955123

Change-Id: Ia83e7bf701fe013a2d4935e7e779783e407a104b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2279254Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Commit-Queue: Maksim Ivanov <emaxx@chromium.org>
Auto-Submit: Leo Lai <cylai@google.com>
Cr-Commit-Position: refs/heads/master@{#784846}