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This complements the existing *ASCII methods, and makes it easier to
append a constant extension to a FilePath. Chrome Remote Desktop will use
this for in-progress file transfers.

Change-Id: Iac84f4809eb9bbbc399f3b5d3322978a7b7174b9
Reviewed-on: https://chromium-review.googlesource.com/c/1330342
Commit-Queue: Erik Jensen <rkjnsn@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611430}