- Eliminates CommitRegion and UncommitRegion methods, replacing them with
  calls to SetPermissions.
- Makes a similar change to the API of VirtualMemory.
- This changes system calls from mmap to mprotect on most POSIX platforms.

Bug: chromium:756050
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: Ib10f8293c9398c6c1e729cd7d686b7c97e6a5d75
Reviewed-on: https://chromium-review.googlesource.com/769679Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49552}