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The restriction is only required when "temporary file" permissions
have been granted, since that is what implies no access to
pre-existing files.

Fixes the issue hit by
  https://chromium-review.googlesource.com/c/chromium/src/+/818486

in that the network service may try to ensure a file is present via
a single call with O_CREAT without knowing if it pre-exists.

Change-Id: I3cf3f2389a78a5a46f43505a64d106614e4dcf90
Reviewed-on: https://chromium-review.googlesource.com/820611
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523527}