- 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}