WebAssembly needs to be able to allocate memory with guard regions, which
requires more functionality from the array buffer allocator. This change adds
functions for reserving memory regions and changing the memory protection.

This CL also includes some minor refactoring of the code to free array buffers.

Bug: chromium:720302
Change-Id: Iab9a266003043b0d36592a79668d1eea53952abf
Reviewed-on: https://chromium-review.googlesource.com/506377Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Eric Holk <eholk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45407}