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This is not strictly required for issue 797292 (filesystem:
support with network service), but it should make that implementation
slightly more efficient and better to land in its own CL.

Bug: 797292
Change-Id: Ia6a20ded0f0c414d632530805bc3ae3eb3053328
Reviewed-on: https://chromium-review.googlesource.com/960922Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Commit-Queue: Chris Mumford <cmumford@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543380}