GitLab

Adds the following abstractions:
- MemoryRegion: Simple Address and size pair.
- PageMemory: Abstraction holding an overall reservation and some
  writeable area.
- PageMemoryRegion: Either a contiguous area of normal pages or an
  area for a single large page. Creates guard pages around writable
  regions.

Guard regions are always added, independently of whether the provided
allocator can actually commit them properly and capped at a maximum of
16KiB. In practice this will result in all platforms except PPC
getting protected regions and PPC only getting an unprotected region.

Drive-by:
- Remove multi-threaded handling of PageMemoryRegion as all data
  structures are thread local and no regions are shared across threads.

Change-Id: I1232216b13ce51482676b16a467dd97d141ac4c7
Bug: 1056170
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2083304
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747210}