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Currently, nothing passes in anything other than a default constructed
one. Consumers will be updated one-at-a-time in followup CLs, and the
default empty NetworkIsolationKey will be removed from method
declarations.

Bug: 966896
Change-Id: If5cc69299de2b0fc6886c304710ffa40423f9536
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1672147
Commit-Queue: Matt Menke <mmenke@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Alex Ilin <alexilin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680037}