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This interface will be used for POSIX stack copying in a following CL.

Bug: 988579
Change-Id: Iefd9c14031ef34f88587de4615b58faa8a58dd9a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1860150
Commit-Queue: Mike Wittman <wittman@chromium.org>
Reviewed-by: Charlie Andrews <charliea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708345}