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There are two implementations of SharedMemory on OSX: Mach and POSIX. Some
consumers of SharedMemory currently require the POSIX implementation. The
default is going to become Mach. This CL updates some consumers of SharedMemory
to force the construction of a POSIX fd-backed SharedMemory region.

BUG=466437

Review URL: https://codereview.chromium.org/1420503010

Cr-Commit-Position: refs/heads/master@{#360502}