This reverts commit 0b0bfc4b.

Reason for revert: Breaks Chromium, see https://crrev.com/c/741484. Already reverted https://crrev.com/c/740083 before this.

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}

TBR=bbudge@chromium.org,ulan@chromium.org,hpayer@chromium.org,mlippautz@chromium.org

Change-Id: If4f62beec765e93c3618bd4824e2c1d4f6c0af4c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:756050
Reviewed-on: https://chromium-review.googlesource.com/741762Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Commit-Queue: Sergiy Byelozyorov <sergiyb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49003}