This is a reland of 0b0bfc4b
Original change's description:
> [Memory] Unify POSIX memory management calls.
> 
> - Moves base::OS memory management calls into platform-posix.cc,
>   using preprocessor to adjust for each platform.
> 
> Bug: chromium:756050
> Change-Id: I2af4dce4379ad1fe9e22e5ab5c6d6a7faa3655b3
> Reviewed-on: https://chromium-review.googlesource.com/738890
> Commit-Queue: Bill Budge <bbudge@chromium.org>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Reviewed-by: Hannes Payer <hpayer@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#48981}

Bug: chromium:756050
Change-Id: Id644d12ee117cdb2eb9d4bf0b4a2c8e3c43ae7be
Reviewed-on: https://chromium-review.googlesource.com/741485Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49011}