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This re-adds POSIX shared memory support on Mac. This
is for use in the sharing of field trials using shared memory. You
can see the design doc for this project at:
https://docs.google.com/document/d/1mTjD99PchmCGYtLvpxaXFObVnid1zmXIsWBI7GjuJgo/
and the motivation for this change under "Port to other platforms".

NOTE: turning on NOPRESUBMIT because this re-adds some banned calls to AllowIO.

BUG=671228
NOPRESUBMIT=true

Review-Url: https://codereview.chromium.org/2555483002
Cr-Commit-Position: refs/heads/master@{#437709}