This CL adds the SharedMemory structure for managing memory attached to
a file descriptor, which can be sent to external processes.
TEST=cargo test
BUG=chromium:738638
Change-Id: I2b6a10e0e7275367ddb578831a73de6956d231bb
Reviewed-on: https://chromium-review.googlesource.com/562509
Commit-Ready: Zach Reizner <zachr@chromium.org>
Tested-by: Zach Reizner <zachr@chromium.org>
Reviewed-by: Dylan Reid <dgreid@chromium.org>
