Port 2cbfa244

Original Commit Message:

  [Memory] Use madvise on POSIX to allow OS to reclaim memory.

  - Use madvise when setting no permissions on memory.
  - Move platform specific mmap flag calculations to a helper fn.

  Bug: chromium:756050,chromium:788341
  Change-Id: I7d420a0abee9656a57fb0317301322da2fd7d7b5
  Reviewed-on: https://chromium-review.googlesource.com/790932


Change-Id: I5f7957066d0be96bd429b3d55c9293ffb996750c
Reviewed-on: https://chromium-review.googlesource.com/804554Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49961}