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The prior two values are not considered to be errors, and will be
mapped to FILE_ERROR_FAILED. Adding the DCHECK to detect incorrect
use of this function.

Bug: None
Change-Id: Id8dcfa01bf18010fc1a59fc20defb907475aac6c
Reviewed-on: https://chromium-review.googlesource.com/798050
Commit-Queue: Chris Mumford <cmumford@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521234}