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With this CL, mini_installer now holds the files it creates open with
FLAG_DELETE_ON_CLOSE so that they are automatically deleted by the
filesystem when they are no longer needed.

The most risky operation is the multi-step process to drop write
permission on the files so that they can be opened for consumption by
parties that do not allow writers. New process exits codes have been
introduced to track whether or not this operation fails in practice.

BUG=516207

Change-Id: Ic5e25692cf3dca0fcc7cd01faf5759648f5c6890
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2307250Reviewed-by: Will Harris <wfh@chromium.org>
Commit-Queue: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790735}