- Change VirtualMemory to match OS memory concepts. Rename Release
  Free, ReleasePartial to Release.
- Adds comments to make the semantics clear. Right now V8 munmaps
  on POSIX, making address space available, while on Windows it is
  only possible to decommit.

Bug: chromium:756050
Change-Id: I6ba04d857ab9e1ca1f273e9e766e0825e67210cc
Reviewed-on: https://chromium-review.googlesource.com/783513Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49586}