- Adds SetPermissions method which returns bool result.
- Eliminates Guard, SetReadAndWritable, SetReadAndExecutable, and
  SetReadWriteAndExecutable methods.
- Adds some Fuchsia memory allocation implementation.
- Some minor fixes in usage of OS::AllocatePageSize and
  OS::CommitPageSize.
- Adds DCHECKs for sanitizing parameters to OS::Allocate/Free.

Bug: chromium:756050
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I966ec6f029dd0371d70eca20bae197d87956f8b5
Reviewed-on: https://chromium-review.googlesource.com/760657
Commit-Queue: Bill Budge <bbudge@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49430}